Update implemented spec and spec document handling

Forbidden aggregates was added in microversion 1.32 (part of Train)
and allocation candidates mappings was added in 1.34 so those specs
have been moved to implemented.

An _extra/.htaccess is added to redirect the approved spec to the
implemented spec. If we're experiencing enough volume of specs to
warrant automating this, it should be easy to do, but at this point
we don't have, nor intend to have, large numbers of specs.

While doing that I noticed the management of spec documents can be
simplified somewhat by moving placeholders and templates to a
non-release specific location. The placeholder can then be used
as a symlink when in approved or implemented when those are first
created or otherwise empty (an empty directory when doing a table of
contents glob is an error so some kind of file is needed).

Contribution docs are updated to reflect the location of the
template.rst file. The template.rst file is updated to be release
generic.

Change-Id: Icb886d5062a52bfc757ed7bbe36ed8a63abe1387

doc/source/_extra/.htaccess

@@ -0,0 +1,2 @@
+redirectmatch 301 ^/placement/([^/]+)/specs/train/approved/2005297-negative-aggregate-membership.html /placement/$1/specs/train/implemented/2005297-negative-aggregate-membership.html
+redirectmatch 301 ^/placement/([^/]+)/specs/train/approved/placement-resource-provider-request-group-mapping-in-allocation-candidates.html /placement/$1/specs/train/implemented/placement-resource-provider-request-group-mapping-in-allocation-candidates.html

doc/source/conf.py

@@ -114,6 +114,7 @@ html_theme = 'openstackdocs'
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
+html_extra_path = ['_extra']
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.

doc/source/contributor/contributing.rst

@@ -279,8 +279,9 @@ If a spec is required there are some guidelines for creating one:
 
      docs/source/specs/train/approved/200056-infinite-resource-classes.rst
 
-  More details on how to write a spec are included in a ``template`` file found
-  in the ``approved`` directory of each release cycle.
+  More details on how to write a spec are included in a ``template.rst`` file
+  found in the ``doc/source/specs`` directory. This may be copied to use as the
+  starting point for a new spec.
 
 * Under normal circumstances specs should be proposed near the beginning of a
   release cycle so there is sufficient time to review the spec and its

doc/source/specs/index.rst

@@ -35,3 +35,9 @@ In Progress
    :glob:
 
    train/approved/*
+
+.. toctree::
+   :hidden:
+
+   placeholder.rst
+   template.rst

doc/source/specs/placeholder.rst

@@ -16,4 +16,4 @@ Placeholder
 ===========
 
 This is a placeholder document until the first specification has been
-implemented.
+created in approved or moved to implemented.

doc/source/specs/template.rst

@@ -342,5 +342,5 @@ understand how the spec has changed over time.
 
    * - Release Name
      - Description
-   * - Train
+   * - <Replace With Current Release>
      - Introduced