Add link checking workflow

Summary

This PR adds automated link checking to the CI workflow to detect and report broken or invalid links in markdown files.

Changes

• Added a new check-links job to the continuous integration workflow
• Integrated lychee action to validate all links in README.md
• Configured to accept valid HTTP status codes (200, 204, 429)
• Set fail: false to prevent workflow failures while still reporting broken links

Benefits

• Early detection: Catch broken links before they affect users
• Automated validation: No manual checking required
• Non-blocking: Workflow continues even if broken links are found (reports them in the logs)
• Comprehensive: Checks all markdown files automatically