Doc generator for Soroban contracts (`cargo doc --document-private-items` published to docs.rs)

[snowrugar-beep]

• Problem Statement. No published API doc; readers need to
  read each lib.rs.
• Why it matters. Open-source maintainability.
• Expected Outcome. contracts/<name>/Cargo.toml ships the
  metadata needed for docs.rs; CI verifies cargo doc builds.
• Acceptance Criteria.
  1. CI verifies docs build clean (no broken links).
• Files / Modules. contracts/*/Cargo.toml,
  .github/workflows/contracts-docs.yml.
• Difficulty. Easy.
• Effort. 1 day.
• Labels. ci, contracts, docs.

[OluRemiFour]

Hi Maintainer,
Interested in working on this issue. I'll implement the requested changes and ensure everything is properly tested before submission.

Payout wallet (Stellar): GALHFNUOYOY4S5CNRK7OI7NKS7UNWYGSJII2POQ6XQKWMUVLKZ2SAWMP GrantFox OSS campaign — smart escrow payout on merge.

[chuks68]

I would like to work on this issue because i am confident i can resolve the issue, having worked on and resolved similar issues. my eta is 4 hours

[grantfox-oss]

🦊 GrantFox — @chuks68 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #162)
2. Your PR will be reviewed by the VertexChainLabs maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@chuks68's PR #224 was approved and merged by @snowrugar-beep.

🏆 @chuks68: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,049 total points). Track your full progress on GrantFox.

👏 Great work, @chuks68! Keep contributing to VertexChainLabs.