docs(contracts): add docs.rs metadata and workflow verification

[chuks68]

Closes #162 This PR resolves the issue of missing published API documentation metadata and verification for all Soroban smart contracts in contracts/.

Changes Included

• Crate Metadata: Configured [package.metadata.docs.rs] and added package metadata (description, repository, license) across all contract packages:
  • contracts/batch-wallet/Cargo.toml
  • contracts/gist-registry/Cargo.toml
  • contracts/governance/Cargo.toml
  • contracts/multisig/Cargo.toml
• Workspace Metadata: Added shared repository and license fields in contracts/Cargo.toml ([workspace.package]).
• CI Workflow: Created .github/workflows/contracts-docs.yml which executes cargo doc --locked --no-deps --workspace with RUSTDOCFLAGS="-D warnings" to fail CI on documentation errors or broken links.

Verification

• Verified local documentation generation with $env:RUSTDOCFLAGS="-D warnings"; cargo doc --locked --no-deps --workspace. Documentation for all 4 contracts built cleanly with zero warnings/broken links.

[snowrugar-beep]

Heads-up: CI was green a moment ago but this PR is no longer marked MERGEABLE, so I didn't auto-merge it. Could you rebase against main and re-push? \xf0\x9f\x99\x8c

[snowrugar-beep]

Quick follow-up: my earlier \xe2\x80\x98rebase\xe2\x80\x99 note was a false alarm \xe2\x80\x94 transient issue on my side. \xe2\x9c\x85 Merged now. docs.rs metadata is much appreciated \xe2\x9c\x93 \xe2\x80\x94 makes the contracts much more discoverable.