docs: add and fix crate-level documentation for all CosmWasm crates

[GarmashAlex]

Fix #2234

This commit adds or updates crate-level documentation comments (//! ...) for all main CosmWasm crates in the repository.

• Each crate now includes a brief description of CosmWasm, its purpose, and a link to the official documentation at https://docs.cosmwasm.com.
• For internal or derive crates, the documentation clearly states that they are for internal use only.
• All crate-level doc comments are placed at the very top of each file to comply with Rust linter requirements and avoid documentation-related linter errors.
• This improves discoverability, consistency, and onboarding for new users and contributors.

[chipshort]

Nice stuff! I put some comments inline.

[GarmashAlex]

@chipshort So sorry for late answer, corrected docs