Add Mkdocs as documentation framework

[scottstanie]

Copying over the basic setup of documentation from dolphin. It uses MkDocs, and the MkdocStrings for autogenerating the reference material. Once you install the requirements in docs/requirements.txt, you can locally view the docs with mkdocs serve:

Other features:

• The setup of the markdown files right now is nudging future documentation towards this: https://diataxis.fr/ which I found via the NEP describing how they make sure nice documentation
• You can cite using the references in docs/references.bib. So in docstrings or other markdown docs, we can add @[Pepe2006ExtensionMinimumCost] and it'll look like this:

- You can make a jupyter notebook, store it in `docs/notebooks/`, and have it rendered in the docs. [See here for an example](https://dolphin-insar.readthedocs.io/en/latest/notebooks/walkthrough-basic/)

[piyushrpt]

LGTM