Add documentation link checking and fix broken links

[iimpulse]

Summary

This PR addresses issue #313 by implementing comprehensive link checking for documentation and fixing several broken links.

Changes Made

• Added GitHub workflow for automated link checking using sphinx-build -b linkcheck
• Fixed malformed VRS URL in variant.rst (removed duplicate https://)
• Fixed obsolete GitHub organization reference in version2.rst
• Updated sphinx configuration to use modern CSS API and set language to 'en'
• Configured linkcheck to run on push/PR to docs, weekly schedule, and manual dispatch

Link Check Results

The link checker identifies ~109 broken/redirected links across the documentation, including:

• Multiple broken HPO links (hpo.jax.org/app/browse/term/*)
• Broken protocol buffer documentation anchors
• SSL certificate issues with some external sites
• Various redirects that could be updated

Test Plan

• [x] Sphinx linkcheck runs successfully and identifies broken links
• [x] Fixed links now work correctly
• [x] GitHub workflow validates properly
• [x] Documentation builds without errors

The workflow will now automatically check for broken links on documentation changes and run weekly to catch external link breakage.

🤖 Generated with Claude Code