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.