Refurbish the documentation

[chandryan]

Description

The documentation is currently in a single file in docbook format. It would be nice to have it in a more "edit-friendly" format and structured across multiple files.

Ideas

The Boost documentation guidelines might be helpful.
I personally had good experience using Sphinx for documentation. It has many directives to dynamically put content into the docs and should also support integrating an API documentation generated by Doxygen.