Add markdown link CI check

[lalitb]

Changes

Shamelessly copied from https://github.com/open-telemetry/opentelemetry-specification/blob/5d9cef817415a15aaa4883e184a38adec79096ed/.github/workflows/checks.yaml#L36, and fixed broken links.

This will recursively check all the markdown files (ignoring CHANGELOG.md), and fail if any dead link is found.

If a CI check isn't necessary, I can modify the PR to exclude it, focusing solely on the updates to the broken links.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (59f5371) 65.8% compared to head (841a123) 65.8%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1540   +/-   ##
=====================================
  Coverage   65.8%   65.8%           
=====================================
  Files        140     140           
  Lines      19505   19505           
=====================================
  Hits       12841   12841           
  Misses      6664    6664           

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