Move workhorse files into final directory

This merge request continues the work of moving the Workhorse docs from /workhorse into a deep subfolder of the doc/development directory. (#354845 (closed))

• Initially, I moved all the files directly to their final locations. However, the directory structure in /workhorse was way more complex than what this feature actually needs.
• I created our standard redirects for the old pages, even though I have a strong suspicion our script will NOT pick them up because they're outside the doc/ directory. It'll cause tech debt, but at least we have functioning redirects in place.
• I combined some of the smallest pages back into the index page, and updated the redirects accordingly.
• I flattened the directory structure - we don't need 3 subdirectories.
• I emphatically did NOT do a cleanup on any of these pages. This merge request incurs a long pipeline because these pages aren't in the doc/ directory. I'll save all other edits and polishing until this work is merged. Changes after that point incur a shorter pipeline.

Remaining work

• Search a few common repos, including www-gitlab-com, for links to these docs. Update them.
• Create a follow-up merge request in the gitlab-docs repository to add these pages to the left nav.
• Cross-link into this folder from doc/development/index.md. Refer to !68538 (diffs) to know where to add the cross-link.

Related issues

• Related to Documentation review: Workhorse (#354845 - closed)
• Related to !68538 (merged) where I cleaned up the linting errors. These files were not receiving any kind of docs-related linting in their old location, and sure enough, they would've broken CI had I moved them without prior cleanup.