Sphinx-generated docs for Zeppelin project

It's a part (or pre-request) of [ZEPPELIN-64](https://zeppelin-project.atlassian.net/browse/ZEPPELIN-64) but deals with how do we manage the project documentation itself, which should be close to code (and evolve together)

The idea is: to update docs you 
 - build docs .rst -> html as a part of the build 
 - then copy the result to GithubPages