fix: emit absolute URLs in llms.txt, llms-full.txt, and sitemap

[lhagenWP]

Why

llms.txt and llms-full.txt are fetched as standalone documents, but
wrote their links as root-relative paths. The site is served under
/docs, so every one of those links resolved against the host root and
404'd. A bot following llms.txt reached nothing.

sitemap.xml from the same build already wrote fully absolute URLs, so
this was a generator inconsistency rather than a platform limit.

What Changed

Every link in llms.txt and llms-full.txt is now absolute under
https://api.whitepages.com/docs. In llms.txt that means the page
targets; in llms-full.txt, the markdown targets and HTML href
attributes carried in from the MDX bodies are rewritten as the text
leaves the app.

Content files keep writing root-relative links — Next resolves those
against basePath for in-app navigation, so the rewrite belongs at the
route boundary, not in the MDX.

sitemap.ts and both routes now share one base-URL constant.

Also in this change

sitemap.xml drops changefreq and priority from every entry, which
Google ignores.

It also drops a hardcoded root entry that duplicated the index page —
the sitemap listed both /docs and /docs/. The index is emitted
without the trailing slash, since /docs/ normalizes to /docs with a
308 before the 307 on to getting-started, and the bare form resolves in
one hop.

Verification

Checked against a built server (next build, then next start):

Output	Root-relative links	Result
llms.txt	109 → 0 markdown	63 targets, all absolute
llms-full.txt	35 → 0 href	144 absolute URLs
sitemap.xml	—	62 entries, no duplicate, no changefreq/priority

Every one of the 63 llms.txt targets was fetched: 62 return 200, and
the index returns a single 307 to getting-started. No 404s. All 62
sitemap entries resolve the same way.

just validate passes.