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/).

Edit:
By the way, some of the links are broken when viewing this branch in GitHub because they are Sphinx-style cross-references between documents (which GitHub doesn't know how to handle) or links to files that will exist when this pull request is merged (e.g. the link in the README to the Getting Started guide.)