Publish stable API documentation with Sourcey and GitHub Pages

[RYDE-PLAY]

Hi, I put together a complete implementation for #395.

This publishes documentation for the current stable master branch while keeping the existing QCBOR v2 alpha documentation clearly linked and separate.

What this adds:

• a Sourcey site combining the project guide, focused guides, and Doxygen API reference
• Doxygen XML output and public API groups for common types, decoding, encoding, spiffy decode, and buffer utilities
• full-text search across 1,296 guide and API entries
• API source links pinned to the exact build commit
• a GitHub Pages workflow that builds on documentation PRs and deploys after pushes to master
• a stable-documentation link in the README, with the existing Security Theory site labeled as the v2 alpha documentation
• a small API-only style adjustment that removes Sourcey's duplicated Doxygen title and summary

The group additions are documentation comments around the existing public headers; they do not change compiled behavior. Private implementation symbols and qcbor_private.h are excluded from the generated API.

Validation performed:

• pnpm install --frozen-lockfile
• pnpm run docs
• generated-site check: 22 HTML pages, 0 broken internal links, 0 private API pages, and 60 revision-pinned source links
• desktop and mobile navigation, search, and representative function-page checks
• make warn
• ./qcbortest — 76 tests passed, 0 failed
• git diff --check

The Doxygen step still prints the existing undocumented-symbol and obsolete-option warnings from the source and Doxyfile, but completes successfully.

One repository setting is needed after merge: under Settings → Pages → Build and deployment, select GitHub Actions as the source. After that, the included workflow will publish and update the site automatically.

I would be happy to adjust the organization, wording, colors, or workflow split to better match the project's preferences.

Closes #395.