Add missing per-contract README documentation across most crates

[observerr411]

• Location: multiple missing files under contracts/*/README.md
• Problem: Most contracts lack local README files, making API usage, deployment, and constraints hard to discover.
• Resolution criteria: Add a README.md for each contract crate (at minimum: purpose, storage model, public methods, auth model, test/deploy steps).

[Itodo-S]

@Itodo-S has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

please let me work on this issue; i can handle it

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Itodo-S to this issue.