fix(links): update check relative links

[dobromirts]

Fixes #530.

Problem

list.mdx and themes/roundness.mdx linked Button Group as
/components/inputs/button-group, which 404s and check-relative-links passed.

The docs collection is rooted at content/<lang>/components
(docs/*/src/content.config.ts), so components/ is never a URL segment.
#468 moved the topic into xplat, which legitimately changed its URL from
/button-group to /inputs/button-group, but the links were rewritten with the
new file path rather than the new URL. (The same commit updated toc.json
correctly, because toc hrefs really are file paths.)

The check couldn't catch it: resolveAbsoluteLink carried a fallback candidate,
resolve(langRoot, path), encoding that same wrong model. The bad link matched a
real file on disk, so CI went green. That fallback had been there since the
checker landed in #355.

Content

• The 2 links → /inputs/button-group.
• One stale toc href: the Grid → Theming entry pointed at a page that doesn't
  exist in EN and was being silently dropped from the sidebar on all three xplat
  sites.

[ChronosSF]

Verified the description against the repo and ran the new script against a fully generated local tree:

• Both content configs root the docs collection at components, so the /components/... prefix rejection is correct, and the old resolve(langRoot, path) fallback was indeed what let #530 through.
• Astro 7.2.6 strips a trailing /index from slugs, so the index-page candidates match what publishes.
• The toc check mirrors docExists() in src/sidebar.ts (as-written plus .md/.mdx swap), and the not-synced-to-Angular filter matches shouldCopy() in sync-generated.mjs.
• Against my local tree the PR script reports exactly the two /components/inputs/button-group links for Angular and the stale grids/theming-grid.mdx toc entry for the three xplat platforms, while the base script reported 81 Angular "not found" links that were all xplat-authored topics not yet synced locally. The second content root resolves those as intended.
• The jp counterparts need no change: both jp Angular topics already use relative links, and the jp xplat source still has the grid theming page at grids/theming-grid.mdx, so its toc entry is consistent.

LGTM.