docs: Transition to .rst for automation

[yangchoo]

Historically, we've maintained separate formats for documentation,
utilizing markdown for README, CHANGELOG and CONTRIBUTING.

While this is easy, this results in some pain. For example, whenever we
updated the README.md file, we need to make the corresponding changes
to the docs/README.rst file.
Similarly, we did not include the CHANGELOG/CONTRIBUTING file since
those did not easily parse into rst files.

This standardizes all our documentation on .rst, which is a common
approach taken by most Python repos with sphinx-docs.

[yangchoo]

Note that I considered using https://pypi.org/project/m2r/ and https://github.com/readthedocs/recommonmark.

The generally work fine with some fixes to our markdown files, however this approach doesn't allow for relative links due to an outstanding issue (see sphinx-doc/sphinx#7000).

Until that issue is resolved, the usual workaround for that is to check-in a symlink, but that doesn't work for all environments (windows for example). That said, the m2r repo seems relatively abandoned.

Alternatively, I figured changing to standardizing on .rst isn't a terrible idea, given that its used elsewhere in the repo and reduces the required dependencies. GitHub has native rendering support for rst, and this approach is relatively common in other Python repos.

[yangchoo]

For the changelog, note that the naiive approach of using pandoc resulted in a bunch of undesirable results, so I had to change the underlying md file quite significantly before running pandoc. This included fixing a couple of entries.

For contributing, all I did was run pandoc and fix some code-blocks.

For readme, it was similar, pandoc with some minor fixes for links.

[rhysormond]

Did a skim of each of the updated files (not super easy to diff effectively). I assume things were largely copied over before reformatting so given that the formatting looks fine it LGTM.

I'd check in on the conf.py copyright though before committing that.

[dauglyon]

Another option here is to move all of the documentation into the docs. I believe GitHub knows to look for such a folder when rendering the readme and licensing info

[yangchoo]

that's good to know, thanks! I'm inclined to keep this structure (top-level) for now, so that the typical items are relatively more visible/discoverable