Fix Engineering handbook folder structure so CODEOWNERS works better

Following on from #2 (closed), there are a lot of sub-directories directly under /handbook/engineering/ that aren't owned by a particular department. This means that CODEOWNERS will contributors to Eric, when he may not be the right person for them.

While we can also add CODEOWNERS entries for top-level sub-directories, we should also have clear DRIs in terms of departments - if the CODEOWNERS would naturally belong to one department, we can put that in the sub-directory for that department.

There are three possible states for a sub-directory:

1. KEEP
   1. Owned by Eric - it's actually part of Engineering as a whole. Examples: cto-staff, okrs.
   2. Already in CODEOWNERS - may belong to a particular department but not currently a problem. Examples: metrics, open-source.
   3. It's a department (same as the above but more specific). Examples: quality, ux.
2. MOVE
   1. Typically we want the best fit to match as a DRI. Examples: architecture to infrastructure/architecture, principles to development/principles.
3. REMOVE
   1. There shouldn't be many of these, but there might be some! Examples: dev-backend, maybe frontend?

• architecture
  • MOVE to infrastructure. This will need a bunch of updates in the handbook, and some in our product docs.
• automation
  • KEEP as it's cross-functional, but add to CODEOWNERS?
• career-development
  • KEEP.
• coding-at-scale
  • MOVE / REMOVE: this could be folded into architecture.
• compensation-roadmaps
  • KEEP.
• critical-release-process
  • REMOVE: this already moved, just make sure it's got a redirect in place and no existing links.
• cto-staff
  • KEEP.
• demos
  • KEEP?
• dev-backend
  • MOVE contents and REMOVE the directory.
• development
  • KEEP.
• emergency-meeting-protocol
  • KEEP.
• engineering-allocation
  • KEEP.
• error-budgets
  • KEEP as it's in CODEOWNERS.
• fast-boot
  • KEEP.
• fellow
  • KEEP.
• frontend
  • KEEP as it's in CODEOWNERS, but questionable given the current org structure.
• gitlab-repositories
  • KEEP.
• ic-leadership
  • KEEP.
• incubation
  • KEEP.
• infrastructure
  • KEEP.
• internships
  • KEEP.
• management
  • KEEP.
• metrics
  • KEEP.
• monitoring
  • KEEP as in CODEOWNERs.
• okrs
  • KEEP.
• open-source
  • KEEP.
• ops
  • REMOVE? Redirects already seem to be in place, but there is content still here.
• performance
  • KEEP as it's cross-functional?
• performance-indicators
  • KEEP.
• plato
  • KEEP.
• principles
  • MOVE to development.
• projects
  • KEEP.
• quality
  • KEEP.
• readmes
  • KEEP but add CODEOWNERS.
• releases
  • KEEP as it's in CODEOWNERS.
• root-cause-analysis
  • KEEP?
• security
  • KEEP.
• starting-new-teams
  • KEEP.
• ux
  • KEEP.
• volunteer-coaches-for-urgs
  • KEEP.
• workflow
  • KEEP as it's in CODEOWNERS.
• workload-management
  • REMOVE? I can't find any links. Otherwise, MOVE to infrastructure as this does not apply to Development.