Raw on-domain .md uses source-relative links that 404 when fetched

[armcconnell]

The per-page Markdown emitted by hooks/emit_markdown.py (e.g. /setup/index.md) keeps MkDocs source-relative links like [Glossary](glossary.md). Fetched at /setup/index.md, that resolves to /setup/glossary.md (404) instead of /glossary/.

MkDocs rewrites these at build time for HTML but not for the raw source we now serve, so agents following links in the .md will miss.

Fix: rewrite intra-doc links to absolute site URLs (or to the sibling index.md) when emitting the Markdown. Follow-up to #176.