Improve description of Jenkins jobs in the documentation

Before major refactoring happen on hudson.openmicroscopy.org.uk in the build/testing workflow, this PR tries to capture some of the existing knowledge on the existing jobs:

- split the jobs description into 3 sections (OMERO, Bio-Formats, Documentation).
- add the OME model doc jobs and the OMERO ice34 jobs
- improve description of doc jobs environment variables

/cc @melissalinkert, @hflynn