Reorganise and update the settings documentation

This commit is primarily a reorganisation fo our settings documentation.
The aim is to move away from the horizon / openstack_dashboard / django
distinction, which makes little sense to non-devs. This will be replaced
by a split between general horizon settings, service-specific settings,
and upstream Django settings. as well as organising the settings in
each section alphabetically. The aim is to promote discoverability of
settings for operators.

This patch also improves visiblity of deprecation, adds missing
tags (versionadded, seealso, deprecated, versionchanged), and
updates content where necessary. Some missing settings were added,
and many settings were also updated to reference other relevant
settings.

Change-Id: Idd496af7d4fe7d242772bb402d22c54c18f05d51
Implements: blueprint pike-docs-overhaul

doc/source/contributor/quickstart.rst

@@ -196,11 +196,6 @@ To add a new dashboard to your project, you need to add a configuration file to
 ``openstack_dashboard/local/enabled`` directory. For more information on this,
 see :ref:`pluggable-settings-label`.
 
-There is also an alternative way to add a new dashboard, by adding it to
-Django's ``INSTALLED_APPS`` setting. For more information about this, see
-:ref:`dashboards`. However, please note that the recommended way is to take
-advantage of the pluggable settings feature.
-
 URLs
 ----
 

releasenotes/notes/bp-pike-docs-overhaul-b4f01f45ced58e07.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - Most of the documentation has been reorganised and updated, or rewritten
+    entirely, with the aim of promoting discoverability and ensuring all
+    content is relevant and up to date.