fix(seo): remove deprecated docs sitemap hints

[hasanalituran]

Summary

• Remove deprecated changefreq and priority sitemap hints from the Docusaurus output.
• Preserve canonical lastmod generation from source document timestamps.
• Keep the existing 34-URL docs sitemap and existing 404 noindex postbuild behavior.

Evidence and affected URLs

• Current live sitemap: https://docs.testream.app/sitemap.xml
• The current live sitemap contains 34 URLs and emits changefreq and priority for every URL.
• The installed SEO sitemap analyzer reported changefreq: 34 and priority: 34; the Testream website sitemap at https://testream.app/sitemap.xml has no deprecated tags.
• The change affects the generated docs sitemap, including https://docs.testream.app/, https://docs.testream.app/getting-started/quick-start, and https://docs.testream.app/reporters/vitest.

Validation performed

• npm run build passed in the isolated worktree.
• npm run postbuild passed and injected the existing noindex tag into build/404.html.
• Generated build/sitemap.xml contains 34 <loc> entries and no <changefreq> or <priority> tags.
• npm run test:indexnow passed: 3 tests.
• npm run typecheck passed.
• git diff --check passed.

Risks and data gaps

• This is a sitemap metadata cleanup; it is not expected to change page copy, canonical URLs, indexing eligibility, or the URL set.
• GitHub Pages deployment and provider recrawl have not been performed in this PR.
• Local validation required an explicit npm run postbuild because this environment has npm config ignore-scripts=true; CI should use its normal script policy.

Manual tasks for Hasan

• Review and approve the focused sitemap configuration change; no copy approval is required.
• Merge and deploy the docs branch through the normal GitHub Pages workflow.
• After deployment, verify https://docs.testream.app/sitemap.xml returns 200, still lists 34 canonical URLs, and no longer emits changefreq or priority.
• Request or allow the normal Bing/Google recrawl; no provider UI action was taken by this PR.
• No credential or admin action is required.
• Perform a quick manual QA check of the docs homepage and one reporter page after deployment.