Doc reorganization

Following the merge of #296, this PR reorganizes the documentation as follows:
- creation of an `omero/` folder where all the OMERO documentation is moved
- creation of a `common/` folder for shared items (images, configuration, themes)
- extraction of shared configuration settings into `common/conf.py`
- creation of a main `Makefile` to build the HTML & PDF versions of both documentations.

To test this PR, 
- check the build is still green (after configuration modification)
- check the HTML & PDF output for both documentations are unchanged