Create separate Middleman instance for the Handbook (Part 2: remaining dirs)

(NOTE: this is Part 2 of this change, following up on Part 1 (#6689 (closed))

Who

The Static Site Editor Group and Brand and Digital Design team are working together to create monorepo structure for www-gitlab-com.

What

We are creating a distinct Middleman build for just the handbook. By isolating the handbook into a separate middleman build, we can start to develop handbook centric features for just this part of the site.

This issue will involve moving the rest of the directories under handbook (the engineering and marketing dirs were moved in Part 1 ((#6689 (closed))

Why

We are doing this so the handbook becomes isolated from other parts of the website and deployment times are reduced. This is also a required step toward creating a true monorepo structure for www-gitlab-com

Tasks

• Move remaining directories under source/handbook to separate build, but only symlinked at first UPDATE: we just moved them, didn't do the extra initial symlink step. E.g. all the files in PartialBuild 6/9. See MR for detailed technical tasks breakdown: !53777 (merged)
• Give people notice that the files will be moving for details).
• Ensure that we update docs for obtaining handbook word/page counts. See !53726 (merged) and !53680 (merged). Done here: !54580 (merged)

Notice Text

Hello! As part of the migration to a monorepo approach for `www-gitlab-com`, we will be moving the rest of the pages under `source/handbook` folder under `sites/handbook/source/handbook. This will happen on this WEEKDAY, MONTH DAY after TIME:00 UTC. 

If you have open MRs or branches which have modified files in these directory, please finish and merge them before then, or be prepared to resolve conflicts before you can merge. If you need help resolving conflicts, or have other questions, please ask on Slack in #mr-buddies or in #handbook.  More details in the issue and MR below. Thanks!

https://gitlab.com/gitlab-com/www-gitlab-com/-/issues/7912
TODO: Add MR link