Split continuous integration section into multiple subsections

- Create one subsection per category of jobs (OMERO, Bio-Formats, docs)
- Preserve continuous-integration page as the top-level menu for CI
- Move some common substitutions to conf.py
- Fix main index
Add description of the breaking label and job naming scheme
Move breaking jobs to special section
Document latest OMERO-5.1-breaking-upgrade job
Add Bio-Formats breaking jobs and make references to breaking sections
Add note about dev_4_4
Add trailing punctuation
Fix labelled spelling
Fix development branch paragraph
to -> with
Document new BIOFORMATS-5.1-breaking-repository-subset job
Fix typos
Merge pull request #699 from sbesson/ci_restructuring

Contributing: CI restructuring