Setup docs to build using Sphinx

This series of commits sets up the docs to build using Sphinx. I also reorganized the documentation somewhat:

- I added a Getting Started guide and moved installation instructions into that guide.

- I moved the man pages into `doc/man/`

Now, running `make` builds the man pages (as it did before) as well as an HTML documentation website. After you run `make`, the main page of the HTML documentation is located at `doc/_build/html/index.html`.

If this looks good to you, it will also be easy to host the docs online for free using [Read the Docs](https://readthedocs.org/).