Improve description of Jenkins jobs in the documentation (rebased onto develop)



This is the same as gh-350 but rebased onto develop.

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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