BF docs: Jenkins cleanup

This PR mirrors the CI cleaning effort done in https://github.com/openmicroscopy/ome-documentation/pull/777 on the Bio-Formats front:
- cleans all references to http://ci.openmicroscopy.org within the Bio-Formats documentation
- redirects all links to the BF artifacts to the download components, i.e. http://downloads.openmicroscopy.org/latest/bio-formats5/artifacts/
- redirects all Javadoc links to the released Javadocs, i.e. under http://downloads.openmicroscopy.org/latest/bio-formats5/api/
- cleans `docs/sphinx/conf.py`
- add a `sourcedir` extlink to point at GitHub directories
- cleans a few redirected links
/cc @hflynn

To test this PR,
- check the documentation sets builds as usual 
- check all URLs are still working and functional
- check there is no more link pointing at ci.openmicroscopy.org