[Contrib Guide] Backport section to RST & final edits

This patch:
- translates the below wiki page to RST:
  https://wiki.openstack.org/wiki/Documentation/HowTo#Git_commit_messages_and_backports
- creates a new folder called Additional Git workflow and moves the file
  cherry-pick.rst to this folder;
- replaces '_' with '-' in some file names for consistency;
- changes the files order in the index.html file;
- updates text for the Abstract section in Introduction.

Change-Id: Icd66e1af93a12f1c3fc827ce228001dced9aa065
Implements: blueprint docs-contributor-guide

doc/contributor-guide/source/additional-git-workflow.rst

@@ -0,0 +1,15 @@
+
+.. _git_workflow:
+
+=======================
+Additional Git workflow
+=======================
+
+This section contains additional workflow instructions that you may need
+while contributing to OpenStack documentation.
+
+.. toctree::
+   :maxdepth: 2
+
+   additional-git-workflow/cherry-pick.rst
+   additional-git-workflow/backport.rst

doc/contributor-guide/source/additional-git-workflow/backport.rst

@@ -0,0 +1,27 @@
+
+.. _backport:
+
+Backport a change
+~~~~~~~~~~~~~~~~~
+
+When committing to the master branch, add the ``backport`` line to your
+commit message for potential backport patches (Installation Guide,
+Configuration Reference, and common directory). For example:
+
+* To backport to a single branch:
+
+  .. code-block:: console
+
+     backport: icehouse
+
+* To backport to multiple branches:
+
+  .. code-block:: console
+
+     backport: icehouse kilo
+
+* If no backporting is needed:
+
+  .. code-block:: console
+
+     backport: none

doc/contributor-guide/source/index.rst

@@ -5,9 +5,10 @@ OpenStack Documentation Contributor Guide
 Abstract
 ~~~~~~~~
 
-.. warning:: This guide is a work-in-progress and changing rapidly
-   while we continue to test and enhance the guidance. Please note
-   where there are open "to do" items and help where you are able to.
+This guide provides detailed instructions on the documentation contribution
+workflow and conventions to be considered by all contributors. Please
+follow these guidelines to keep the documentation structure, style, and
+syntax consistent.
 
 Contents
 ~~~~~~~~
@@ -15,21 +16,20 @@ Contents
 .. toctree::
    :maxdepth: 2
 
-   introduction.rst
    first-timers-quickstart.rst
+   additional-git-workflow.rst
+   writing-style.rst
    rst-conv.rst
    docbook-conv.rst
    docs-structure.rst
    content-specs.rst
-   docs_builds.rst
-   build-locally.rst
-   writing-style.rst
-   json_conventions.rst
+   tools-and-content-overview.rst
    doc-bugs.rst
-   blueprints-and-specs.rst
    docs-review.rst
-   tools_and_content_overview.rst
-   cherry-pick.rst
+   docs-builds.rst
+   build-locally.rst
+   blueprints-and-specs.rst
+   json-conventions.rst
 
 Search in this guide
 ~~~~~~~~~~~~~~~~~~~~

doc/contributor-guide/source/introduction.rst

@@ -1,7 +0,0 @@
-
-.. _introduction:
-
-===========================
-Who should read this guide?
-===========================
-