Contributing: CI restructuring

This PR restructures the Continuous Integration page into several subsections to improve its clarity. This includes the following notable changes:
- the existing `continuous-integration` page is preserved but now acts as a top-level menu to the CI sections
- OMERO, Bio-Formats and Docs CI jobs sections are now moved into separate pages in the HTML output
- the scc scripts page is moved to the continuous integration menu (and removed from the index page)
- some information is added in the introductory page about labels (incl. `breaking`) and job naming scheme.

--no-rebase