Prevent broken links on Handbook pages

Problem to solve

Given the interconnected nature of the GitLab handbook, a relatively small change to a popular page can cause a ripple effect and result in many broken links across the handbook. These broken links can go uncaught and unnoticed. Additionally, broken links can result in orphaned pages.

Reference

A recent merge request highlights some of the potential impact: !41873 (merged)

Proposal

Investigate existing link-checking jobs during the build and ensure that the user is getting appropriate feedback about either broken links on the current page, or changes that would break links on other pages. Consider making the rule blocking for merging handbook changes.

This solution should check deeplinks as well.