docs: publish the documentation on zygoon.pl/pkgproxy/

pkgproxy's Markdown documentation (README, DEPLOYMENT, docs/.md and docs/design/.md, NEWS as release notes; not AGENTS.md) is published at https://zygoon.pl/pkgproxy/ by the zygoon.pl site (Astro), which carries pkgproxy as a git submodule and builds from pkgproxy main on every deploy.

On pkgproxy's side this needs:

• a docs-site job that, on main pushes and after dagger-check, triggers zygoon.pl's pipeline (trigger: project: zygoon/zygoon.pl, branch: main), so the site follows main;
• docs/docs_test.go extended from the design docs to every published document: relative links and anchors resolve (GitLab's anchor rule), and no raw <tag> outside code, which the site would render as HTML. The two that exist today (cannot <verb> in cli-and-l10n.md, <ts> in statsapi-internals.md) get backticks.

Along the way: AGENTS.md still calls the repository private (it is public), and the dco job's rule compares against the literal string '$CI_DEFAULT_BRANCH', so it never runs on main.