Handbook restructuring

[Update: removed WIP] [Original comment:] Re-ordered the main handbook page, added a table of contents at topic, and indicated how I intend to re-organize certain sections. Labeled it WIP, so that we don't go live with different versions of the handbook main page until everything is done. @sytses please review and provide comments. \ccc @slykahn Also appreciate your input here since you've been providing a lot of the new content that will now find a new home in the new structure.

My next move is to work on the onboarding page specifically, which will combine a lot of the existing onboarding pages and tasks.