Add a documentation page

1. Create an MDX file under Internal-Documentation/src/content/docs/<section>/your-page.mdx.
2. Add frontmatter: title, description.
3. Follow the documentation standards.
4. Register the page in Internal-Documentation/astro.config.mjs sidebar.
5. Run pnpm dev in Internal-Documentation/ and verify the page renders.
6. Cross-link from related pages (onboarding map, glossary, domain index).

If the page documents new behavior, remove or update any stale references elsewhere (grep for old endpoint names).