doc: Split flavors docs into admin and user guides

There are currently two docs describing flavors in 'admin', which
contain a lot of overlapping information. Fix this by keeping the
configuration guide (how to create, delete, modify flavors) in
'admin', while moving the reference-style parts into 'user'. We
cross-reference the two internally.

Given that large chunks of this needed to be rewritten, we've taken the
opportunity to fix a poor description for the RXTX factor, closing a
longstanding bug in the process.

Change-Id: Ia57c93ef1e72ccf134ba6fc7fcb85ab228d68a47
Closes-Bug: #1688054

doc/source/_extra/.htaccess

@@ -6,6 +6,7 @@
 #   sort
 
 redirectmatch 301 ^/nova/([^/]+)/addmethod.openstackapi.html$ /nova/$1/contributor/api-2.html
+redirectmatch 301 ^/nova/([^/]+)/admin/flavors2.html$ /nova/$1/admin/flavors.html
 redirectmatch 301 ^/nova/([^/]+)/aggregates.html$ /nova/$1/user/aggregates.html
 redirectmatch 301 ^/nova/([^/]+)/api_microversion_dev.html$ /nova/$1/contributor/microversions.html
 redirectmatch 301 ^/nova/([^/]+)/api_microversion_history.html$ /nova/$1/reference/api-microversion-history.html

doc/source/index.rst

@@ -165,6 +165,7 @@ Once you are running nova, the following information is extremely useful.
      This guide was imported during the Pike cycle and is a bit out of
      date. It will be updated during Queens to be more accurate.
 
+* :doc:`Flavors </user/flavors>`: What flavors are and why they are used.
 * :doc:`Upgrades </user/upgrade>`: How nova is designed to be upgraded for minimal
   service impact, and the order you should do them in.
 * :doc:`Quotas </user/quotas>`: Managing project quotas in nova.
@@ -278,6 +279,7 @@ looking parts of our architecture. These are collected below.
    user/conductor
    user/feature-classification
    user/filter-scheduler
+   user/flavors
    user/placement
    user/quotas
    user/support-matrix