…KEN)

Production was returning 500 for /docs-sitemap.xml because the deploy
env has a revoked or invalid GITHUB_TOKEN. Octokit was sending it on
every request, GitHub replied "Bad credentials" (401), the exception
bubbled out of getServerSideProps, and Next rendered the error page.

Two layers of defense:

1. parse-mdx-docs.ts: only pass `auth` to Octokit when the token is a
   non-empty trimmed string. A missing/blank/invalid-because-empty
   token now falls back to unauthenticated GitHub requests (60 req/hr,
   plenty for a sitemap and dev-time doc fetches). Removing the env
   var entirely is now a clean way to recover from a bad token.

2. docs-sitemap.xml.ts: wrap getAllDocUri() in try/catch. If anything
   in the GitHub chain still fails at runtime (rate-limit, network,
   downstream API change), log the error and emit a valid empty
   <urlset> rather than 500.

Verified locally: with no token, the sitemap populates with 51 real
doc URIs. With an invalid token, it returns 200 with an empty but
well-formed sitemap, and the error is logged to the server.

To populate the production sitemap, either set a valid read-only PAT
on GITHUB_TOKEN in the deploy env, or unset it to use unauth requests.