Add documentation for Python changes brought by #1013

[tom-osika]

PR #1013 brings many changes to the existing Python infrastructure and adds new python functionality. When the PR is merged, other branches with Python changes will have to accommodate the changes brought by #1013. To facilitate the process of updating these outstanding branches, documentation should be added addressing the following questions (and possibly others):

• I'm writing Python code that uses KWIVER. How have the modules been renamed?
• I'm writing bindings for a vital type, algo, etc. Where does that go in the source tree now and how do I add it to the build?
• I'm writing a Python arrow, process, etc. Where does that go in the source tree now and how do I add it to the build? In particular, how do I make sure plugin discovery picks it up?

See this comment for more information: https://github.com/Kitware/kwiver/pull/1013#issuecomment-664492346

[tom-osika]

@neal-siekierski