Move documentation to Sphinx / Read the Docs

[scotws]

The custom setup for documentation was fun, but Sphinx (developed for Python documentation) solves these problems in a simply, standardized way and can be combined with Read the Docs.

For an introduction, see http://www.ericholscher.com/blog/2016/jul/1/sphinx-and-rtd-for-writers/

[scotws]

So I've done some experimenting with Read the Docs, and this is how it works as a simple example: https://github.com/scotws/ubuntu-nas is the source and https://ubuntu-nas.readthedocs.io/en/latest/index.html the result on Read the Docs (with a very, very simple theme). ReStructured Text is not half as powerful as AsciiDoc, but the integration is far better, and we can get rid of all kinds of silly shell scripts.