Create some sphinx documentation and host on readthedocs.io

[ehashman]

auditwheel's documentation is sorely lacking, and that gap makes it harder to contribute and use. Hosting some documentation on readthedocs.io would be awesome.

I'd like to see the following documentation written:

• a proper usage guide for the many subcommands (beyond a couple README examples and --help info)
• an examples page, which shows how various projects use auditwheel
• a "why" page, with information on the manylinux policies and links to the relevant PEPs

It might also be nice to add a stretch goal of developer documentation that explains the codebase's layout but maybe let's start small 😄