Add CI workflow for link checking and HTML validation

[jeanmachuca]

What

The website repo has only a deployment workflow (pages.yml) but no CI workflow for quality checks.

Why

• Broken links to GitHub repos go undetected
• HTML/CSS validation issues are not caught
• Changes to index.html or style.css can break without detection

How

Add .github/workflows/ci.yml with:

• HTML validation (validator.nu or html-validate)
• CSS validation (stylelint)
• Broken link checker (lychee or html-proofer)
• Trigger on push and PR to main/development