docs: fix link-checker file discovery (#10348)
### Description
The broken link checker lost its discovery of the MDX files so it wasn't
reporting issues. Also fixes some links that went broken while this
analysis was broken.
### Testing Instructions
[This
run](https://github.com/vercel/turborepo/actions/runs/14514917931/job/40721676278?pr=10348)
shows broken links being discovered again, so we know the root problem
is fixed.
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Co-authored-by: Chris Olszewski <chris.olszewski@vercel.com>
