ci: add per-package rustdoc workflow

[h1065153539-create]

Summary

• Add .github/workflows/rustdoc.yml with a per-package matrix for the root contract crate and quicklendx-contracts/.
• Run cargo doc --no-deps --locked for each package so rustdoc failures are scoped to the package that introduced them.
• Document the PR/main rustdoc validation path in docs/RUSTDOC.md.

Closes #1481

Tests

• git diff --cached --check before commit
• Attempted cargo doc --no-deps --locked in both . and quicklendx-contracts/; local Windows environment cannot complete because MSVC link.exe is not installed. The workflow runs on ubuntu-latest, where this local linker issue should not apply.

Notes

• The existing base branch has known Rust CI failures on src/lib.rs (unclosed delimiter) from earlier PR checks. This PR adds an isolated rustdoc workflow and does not modify contract source.

[h1065153539-create]

CI note: the new Rustdoc workflow is wired and running, but both matrix entries are blocked by existing contract source compile/doc errors rather than the workflow syntax.

Evidence from the job logs:

• cargo doc (root contract crate) reaches Documenting quicklendx-contracts v0.1.0 at the repository root, then fails on src/lib.rs:94:30 with this file contains an unclosed delimiter.
• cargo doc (quicklendx-contracts) reaches Documenting quicklendx-contracts v0.1.0 in quicklendx-contracts/, then fails on existing source errors including duplicate compute_investor_tier, unresolved OptionalDisputeResolution, missing QuickLendXError, and private MAX_QUERY_LIMIT imports.
• This PR changes only .github/workflows/rustdoc.yml and docs/RUSTDOC.md; it does not modify contract source.