test: check site project links against MANIFEST in CI

[lesnik512]

scripts/check_site_content.sh checked the built homepage for a hand-kept list of repos. Nothing in CI or the justfile ran it, and the list had already drifted: it did not include compose2pod, so a site missing that link would have passed.

This replaces the script with a test in tests/test_site.py. The repos in the project lists of docs/index.md must equal MANIFEST minus the modern-di-* integrations, the same rule the profile test uses since #128. It runs in the existing test job and fails on extra links as well as missing ones. It reads the Markdown source rather than the built HTML; mkdocs build --strict in the docs job already covers the build.

Red check: with the compose2pod line removed from docs/index.md the new test fails, and it passes with the line restored. uv run pytest passes locally (175 tests).