Enable commonmark-ish docs via myst-parser plugin

[JordonPhillips]

This adds the myst-parser plugin plugin to the sphinx docs configuration. myst is a markdown flavor that extends commonmark with the ability to call sphinx directives and roles, like so:

.. directivename:: arguments
   :key1: val1
   :key2: val2

   This is
   directive content

```{directivename} arguments
:key1: val1
:key2: val2

This is
directive content
```

Smithy's own doc traits are in commonmark format so I think it makes sense to add this here. I've not attempted to re-write any existing docs as I think that would be a waste of time. With the plugin installed, both rst and markdown are supported simultaneously, differentiated based on the file extension.

I also ran the black formatter on conf.py as part of this.

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