Add "pants developer center" documentation section.

Move developer-focused docs into a "Pants Developer Center" subpage. The
sidebar is getting pretty tall, and this stuff has become large enough to have
its own page. No URLs change - links are just consolidated in a subpage.

Also delete the now unneeded migration guide.

Testing Done:
Generated the site locally and things look good.

Reviewed at https://rbcommons.com/s/twitter/r/70/