Fixup merge resolution: Add and update rewards to open challenges lacking one

Proposes a reward of 10k USD for several challenges and 20k for the
larger ones among them.

This PR restores what model-checking#459 originally proposed and had gotten approved,
but was then lost during merge-conflict resolution.

Author: tautschnig

doc/src/challenges/0001-core-transmutation.md

@@ -4,7 +4,7 @@
 - **Tracking Issue:** [#19](https://github.com/model-checking/verify-rust-std/issues/19)
 - **Start date:** *2024/06/12*
 - **End date:** *2025/04/10*
-- **Reward:** *N/A*
+- **Reward:** *10000 USD*
 - **Contributors**: [Alex Le Blanc](https://github.com/AlexLB99), [Patrick Lam](https://github.com/patricklam)
 
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