Minor documentation update

* Update the index page's description.
* Add a section about the hash ring to architecture.
* Add some new modules to, and remove some old modules from,
  the developer doc quick links, to make the list of quicklinks
  shorter and more useful.
* Correct a few broken links in the developer docs.

Change-Id: Ifac1dc355631a2d77b7ccdea362fb83ffbe6a4db
Update method doc strings in pxe.py

Change-Id: Ib9de6636cf900a02b8f22c0403b0edd3604b1cad
Merge "Minor documentation update"
Merge "Update method doc strings in pxe.py"