fix(docs): keep versioned pages out of search indexes

- Add self-canonical noindex signals and headers for archived docs
- Restore sitemap lastmod dates from the tracked checkout

changes/docs-site-index-hygiene.md

@@ -0,0 +1,5 @@
+type: internal
+area: docs
+
+- Excluded the `/main/` and `/v/<version>/` docs trees from search indexing with a self-referential canonical, `noindex,follow`, and a matching `X-Robots-Tag` header, so crawlers spend their budget on the current docs instead of ~30 archived copies of every page.
+- Restored `<lastmod>` dates in the docs sitemap, which were silently dropped because production builds render from an untracked release snapshot.