Migrate details from hack.md design documentation into Github wiki

[kfrz]

Background

We should keep as much information as possible within the built-in Github wiki, so that developers have quicker access to frequented information. Currently we have a large design document living on hack.md.

This information should be parceled and added to the wiki.

Note: remember to add links to new pages into the sidebar so we can edit the order, add nesting as needed, etc. (Builtin "pages" widget is dumb and doesn't support ordering.)

Definition of Done

• All information captured in this document has been migrated to this wiki.