Update docs

[mwtoews]

There are a number of changes related to the docs. In summary:

• Rename reStructuredText files with *.rst suffix
• Refresh and simplify config.py from re-running sphinx-quickstart with a modern version of Sphinx
• Move docs/source/changes.rst to top-level CHANGES.rst; also move recent changes from README.md to CHANGES.rst
• Use sphinx-issues to link various items in CHANGES.rst to GitHub

There are a few releases in PyPI's history that have no change notes, but these micro releases probably had only minor changes.