Another contributing docs reorganization

Redorder it all into a single docs/contributing/ subdirectory with one table
of contents and no sub-sub-directories (except for the frontend guidelines one,
which still needs sorting out). This makes it much easier to see what's there
(one main table of contents for all of contributing) and navigate.

Add _CKAN issue tracker hyperlink target to conf.py.

Move commit messages guidelines to their own page.

Move reviewing a pull request guidelines to their own page.

Fix title style of various titles.

A few other small changes
Fix a Sphinx cross-reference
Add a couple of notes to the documentation guidelines