Move README content to Sphinx documentation

[glesica]

Most of the content in the README ought to move to the Sphinx documentation (see docs/). The docs are written in RST format, there is a concise tutorial available for this. Pay attention to organization, we want clear "paths" through the documentation for different kinds of users:

• New BAGEL developers
• Benchmark developers
• Benchmark tests (tool developers, etc.)
• People who want help interpreting results

We can also copy content from the slide deck into the documentation.