Add fee transfers documentation and update sidebar

Adds comprehensive fee transfer transaction documentation including:
- Overview and transaction structure
- Single vs. double transfer examples
- Token constraints and validation
- Account creation handling
- Test coverage references

Updates sidebar to include fee-transfers.md under Transaction types.

Changes ported from mina-rust PR #1553.

Author: dannywillems

website/docs/developers/transactions/fee-transfers.md

@@ -0,0 +1,164 @@
+---
+sidebar_position: 3
+title: Fee transfers
+description: Understanding fee transfer transactions in the Mina protocol
+slug: /developers/transactions/fee-transfers
+---
+
+# Fee transfers
+
+## Overview
+
+Fee transfers are protocol-generated transactions that distribute collected
+transaction fees to block producers. Unlike user commands, fee transfers are
+created automatically during block production and do not require user
+signatures.
+
+A fee transfer consists of:
+
+- **Receiver(s)**: One or two block producers receiving fees
+- **Fee amount(s)**: The fee amount(s) to distribute
+- **Token**: Must be the default token (MINA)
+
+## Transaction structure
+
+Fee transfers use a specialized structure that supports one or two receivers:
+
+<!-- CODE_REFERENCE: ledger/src/scan_state/transaction_logic/mod.rs#L357-L357 -->
+
+```rust reference title="ledger/src/scan_state/transaction_logic/mod.rs"
+https://github.com/o1-labs/mina-rust/blob/develop/ledger/src/scan_state/transaction_logic/mod.rs#L357-L357
+```
+
+<!-- CODE_REFERENCE: ledger/src/scan_state/transaction_logic/mod.rs#L330-L334 -->
+
+```rust reference title="ledger/src/scan_state/transaction_logic/mod.rs"
+https://github.com/o1-labs/mina-rust/blob/develop/ledger/src/scan_state/transaction_logic/mod.rs#L330-L334
+```
+
+### Creating fee transfers
+
+Fee transfers are constructed using the `of_singles` method which validates
+token compatibility:
+
+<!-- CODE_REFERENCE: ledger/src/scan_state/transaction_logic/mod.rs#L396-L412 -->
+
+```rust reference title="ledger/src/scan_state/transaction_logic/mod.rs"
+https://github.com/o1-labs/mina-rust/blob/develop/ledger/src/scan_state/transaction_logic/mod.rs#L396-L412
+```
+
+## Transaction application
+
+### First pass
+
+During the first pass (`apply_transaction_first_pass`), the fee transfer:
+
+1. **Validates token compatibility**
+   - All fees must be in the default token (MINA)
+   - Rejects non-default tokens
+
+2. **Processes single or double transfers**
+   - For single transfers: adds fee to receiver's balance
+   - For double transfers: adds respective fees to both receivers
+   - Creates receiver accounts if they don't exist
+
+3. **Deducts account creation fees**
+   - If receiver account doesn't exist, charges account creation fee
+   - Receiver gets: `fee_amount - account_creation_fee`
+
+**Implementation:**
+`ledger/src/scan_state/transaction_logic/transaction_partially_applied.rs`
+
+### No fee payment
+
+Fee transfers do not pay fees themselves - they are protocol transactions:
+
+- No nonce increment
+- No fee payer
+- No signature required
+- Cannot be submitted by users
+
+## Account creation
+
+If a receiver account doesn't exist, it is created automatically. The account
+creation fee (1 MINA by default) is deducted from the fee being transferred.
+
+## Single vs. double transfers
+
+### Single transfer
+
+Distributes fees to one block producer. See
+[test_apply_single_fee_transfer_success](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L94-L95)
+for an example:
+
+<!-- CODE_REFERENCE: ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L94-L95 -->
+
+```rust reference title="ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs"
+https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L94-L95
+```
+
+### Double transfer
+
+Distributes fees to two block producers (e.g., coinbase producer and snark work
+producer). See
+[test_apply_double_fee_transfer_success](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L169-L174)
+for an example:
+
+<!-- CODE_REFERENCE: ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L169-L174 -->
+
+```rust reference title="ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs"
+https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L169-L174
+```
+
+## Token constraints
+
+Fee transfers must use the default token. For double transfers, both tokens must
+match. The validation is implemented in
+[FeeTransfer::of_singles](https://github.com/o1-labs/mina-rust/blob/develop/ledger/src/scan_state/transaction_logic/mod.rs#L400):
+
+<!-- CODE_REFERENCE: ledger/src/scan_state/transaction_logic/mod.rs#L400-L408 -->
+
+```rust reference title="ledger/src/scan_state/transaction_logic/mod.rs"
+https://github.com/o1-labs/mina-rust/blob/develop/ledger/src/scan_state/transaction_logic/mod.rs#L400-L408
+```
+
+See
+[test_apply_fee_transfer_incompatible_tokens](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L333-L337)
+for an example creating fee transfers with incompatible tokens:
+
+<!-- CODE_REFERENCE: ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L333-L337 -->
+
+```rust reference title="ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs"
+https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L333-L337
+```
+
+The validation fails with an error:
+
+<!-- CODE_REFERENCE: ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L344-L346 -->
+
+```rust reference title="ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs"
+https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L344-L346
+```
+
+## Testing
+
+Comprehensive tests are available in
+[`ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs`](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs):
+
+- [`test_apply_single_fee_transfer_success`](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L75) -
+  Single fee transfer to existing account
+- [`test_apply_double_fee_transfer_success`](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L137) -
+  Double fee transfer to two receivers
+- [`test_apply_fee_transfer_creates_account`](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L237) -
+  Fee transfer creating new account
+- [`test_apply_fee_transfer_incompatible_tokens`](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs#L316) -
+  Token compatibility validation
+
+## Related files
+
+- [`ledger/src/scan_state/transaction_logic/mod.rs`](https://github.com/o1-labs/mina-rust/blob/develop/ledger/src/scan_state/transaction_logic/mod.rs) -
+  Fee transfer type definitions
+- [`ledger/src/scan_state/transaction_logic/transaction_partially_applied.rs`](https://github.com/o1-labs/mina-rust/blob/develop/ledger/src/scan_state/transaction_logic/transaction_partially_applied.rs) -
+  Fee transfer application logic
+- [`ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs`](https://github.com/o1-labs/mina-rust/blob/develop/ledger/tests/test_transaction_logic_first_pass_fee_transfer.rs) -
+  Fee transfer tests

website/sidebars.ts

@@ -95,6 +95,7 @@ const sidebars: SidebarsConfig = {
           label: 'Transaction types',
           items: [
             'developers/transactions/coinbase',
+            'developers/transactions/fee-transfers',
           ],
         },
       ],