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
merge develop -D org: PR 626 (Add new Squash tutorial and screenshots)
merge develop -D org: PR 699 (Contributing: CI restructuring)
merge develop -D org

Repository: openmicroscopy/ome-documentation
Updating 3796b79..8435d81
Merged PRs (fast-forward):
  # PR 694 hflynn 'Suggested mailing list note update (rebased onto develop)'
  # PR 697 rleigh-dundee 'limitations: Document fileset log file workaround (develop)'

Merged PRs:
  # PR 626 pwalczysko 'Add new Squash tutorial and screenshots'
  # PR 699 sbesson 'Contributing: CI restructuring'

Generated by OMERO-5.1-merge-push#92 (http://ci.openmicroscopy.org/job/OMERO-5.1-merge-push/92/)