Split formats and OMERO documentations

This PR fulfils a series of requirements as discussed with @hflynn, @qidane, @joshmoore:
- ability to build Formats doc independently of OMERO doc
- possibility to build both docs in one command
- maintenance of both docs under same repo ome-documentation.git

Despite, the impressive number of changed files, most of these changes are `git mv` commands. Below is a summary of major changes:
- new `omero/` folder created where all the OMERO documentation is moved
- images under `images/formats/` are moved to `formats/images` & formats documentation links are fixed
- OMERO `conf.py` split between shared `config/all.py` and specific `omero/conf.py`
- OMERO `themes` split between top-level `themes` (plone theme) and specific `omero/themes` (OMERO sidebar toc...)
- 2 new `Makefile` added: one for Formats documentation and one top-level file which loops over both products docs
- build output is copied to `_build/{output}/{product}` (top-level directory) e.g. `_build/html/omero` or `_build/latex/formats`

Both documentations can be compiled at once from top-level directory with the command
```
SPHINXOPTS="-W -D html_theme=plonematch" make clean html
```

NB: `SPHINXOPTS="-W" make latexpdf` still fails for the formats documentation (local links needing to be fixed)