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