Akihiro Motoki c897a8e444 doc: Reorganize contributor doc
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
2020-08-20 08:34:11 +09:00
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