Document API docs build path

[akmhatey-ai]

Summary

• document where the API docs workflow publishes libhal-picosdk docs and how to run the local Doxygen + Sphinx build
• rename copied docs metadata from libhal-arm-mcu to libhal-picosdk so the generated project/switcher metadata matches this repo

Closes #7

Validation

• $env:LIBHAL_API_VERSION='local'; $env:LIBHAL_LOCAL_BUILD='1'; Push-Location docs; & 'C:\Program Files\doxygen\bin\doxygen.exe' doxygen.conf; uv run --with-requirements requirements.txt sphinx-build -b html . build/html; Pop-Location
• git diff --cached --check
• gitleaks detect --pipe --redact --verbose --no-color on the staged diff

Notes

• The local docs build succeeds with the repository's existing documentation warnings for undocumented picosdk symbols and stale copied Breathe references. I did not try to fix those in this small docs-orientation PR.
• The libhal-picosdk public API path returned 404 before this change; the linked path follows the existing API deploy workflow's libhal.github.io/api/{repo}/{version} convention.