Ussuri community goal "Project specific contributor and PTL docs"
want us to use doc/source/contributor/contributing.rst as an entry
point, but the current contributor/contributing.rst in horizon
contains a lot of information.
Before updating the file to satisfy the community goal criteria,
this commit reorganize the contributor doc.
It splits the existing contents in contributing.rst into pieces.
Change-Id: I8aa9a310a99e0eed5a93ed040a02a2829ee09bf7
c897a8e444