This is the first in a series of changes to the content in
doc/source to make it more accurately reflect the current state of
placement, now that it is in its own repo.
There are small changes throughout the front page, mostly to make
it more clear and make useful stuff a bit more discoverable.
The two major changes are:
* Older (pre-stein) upgrade notes are found by linking back to
nova's rocky documentation.
* The REST API history is moved to its own sub-page, otherwise it
the page is overwhelming and noisy.
Close review is probably required. cdent typed this. That has
implications.
Change-Id: I66e0c7d18b253b0a5a8fdac65e30b5b3cef37db2
176524643d