docs: add irmars repo page, sidebar entry, and overview row

[dobby-coder]

Summary

• New `docs/repos/irmars.md` documents the fork rationale (upstream dormant since 2021; needed reqwest 0.12 + thiserror 2.0), crates.io publication as `irmars`, and the `package = "irmars"` Cargo rename used by postguard so `use irma::...` keeps working.
• Adds `irmars` to the Cryptographic Libraries sidebar group in `docs/.vitepress/config.mts`.
• Adds an `irmars` row to the Cryptographic Libraries table and extends the dependency graph in `docs/repos/overview.md` to show irmars feeding pg-core / pg-pkg / pg-cli.

Closes #102

Test plan

• `npx vitepress build docs` succeeds
• `docs/.vitepress/dist/repos/irmars.html` is generated
• Reviewer quickstart: `git fetch origin && git checkout docs/irmars-page && npm install && npm run docs:dev`

[dobby-coder]

Rule-compliance pass found two items, both fixed in cf62fc2:

• TOML snippet pinned to source. The pg-core/pg-pkg/pg-cli Cargo dep example used version = "0.2", but the actual pg-core/Cargo.toml at f2f06cd5 is 0.2.2. Per the postguard-docs source-link convention (code snippets must come from real code with a commit-pinned link), updated to 0.2.2 and added a <small>[Source: …]</small> link.
• Reviewer assignment. Per the org-wide PR-review rule, every PR needs a reviewer — assigned @rubenhensen.

Build re-verified: npx vitepress build docs succeeds, dist/repos/irmars.html generated.