Create sub-sections for different release series

When looking at the documentation, the release notes section on the
sidebar was incredibly long, due to the number of releases available.
To make this more usable, I've split the release notes index into
sections for each release series (e.g., 3.x, 2.x, etc.). This allows
documentation users to open the release notes sidebar section, and then
select which series they want. That will expand and let them pick the
version.

See merge request !145

Add a man page for Flake8

Document Flake8 for users expecting `man flake8` to work and include the
man page in Flake8's documentation.

Closes #247

See merge request !146

Correct exclude paths in git hook

When running the git hook, even though the appropriate config file is
found and processed, the exclude patterns and paths were not being
corrected to account for the temporary directory that we copy the files
into.

Related-to #223

See merge request !147

When running the git hook, even though the appropriate config file is
found and processed, the exclude patterns and paths were not being
corrected to account for the temporary directory that we copy the files
into.

Related-to #223

Document Flake8 for users expecting `man flake8` to work and include the
man page in Flake8's documentation.

Closes #247

When looking at the documentation, the release notes section on the
sidebar was incredibly long, due to the number of releases available.
To make this more usable, I've split the release notes index into
sections for each release series (e.g., 3.x, 2.x, etc.). This allows
documentation users to open the release notes sidebar section, and then
select which series they want. That will expand and let them pick the
version.

Add documentation for VCS hooks

Closes #249

See merge request !144

Closes #249

Remove reference to OpenStack Swift

This section of the docs could be read as pointing out the project as a
bad example. It was not intended this way, but I would like to avoid
anyone interpreting it this way. As a result, we'll continue to use
their configuration section as an example but not name names.

Closes #228

See merge request !143