feat: Validate external URLs in CI using Lychee

[biru-codeastromer]

Fixes #364

Description:

-This PR implements automated external link validation using lychee in CI to ensure that all external URLs return 200 OK as requested in issue #364.

Changes Made:

✅ Installed lychee using cargo install lychee (if not already installed).
✅ Ran lychee to scan all Markdown (.md) files for broken links.
✅ Identified broken links and validated external URLs.
✅ Re-ran lychee to verify the fixes.
✅ Added a GitHub Action (link-check.yml) to automate future link validation in CI:

• Runs lychee on every pull request and push to main.
• Fails the workflow if broken links are detected.
  ✅ Enforced failure if any external link is broken
  ✅ Now, CI fails and reports broken links automatically

Impact

• Ensures external URLs are valid, improving documentation reliability.
• Automates link checking, reducing manual effort and potential oversight.
• Prevents broken links from being merged, maintaining content integrity.

Testing :

Testing done .

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue