Remove all references to CI jobs in the official documentation (rebased onto dev_5_0)



This is the same as gh-776 but rebased onto dev_5_0.

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Following #772 discussion, I reviewed our usage of `JENKINS_JOB`. The original idea was to be able to point at staging artifacts/Javadoc for review essentially. As it turns out, this seems to add unnecessary complexity to the build system. This PR proposes to reduce the number of envvars by:
- purely removing `JENKINS_JOB` and all its dependent extlinks
- systematically pointing at the downloads pages for releases/ API
NB: if CI links need to be specified we should most likely handle them via our Contributing documentation which has an entire section dedicated to our CI infrastructure.
NB2: while filing out this PR, I noticed we do not redirect http://downloads.openmicroscopy.org/latest/bio-formats5/api/ as we do for the OMERO API documentation. Worth setting up a redirect for this one too? /cc @ajpatterson

Arguably as part of this PR, I could add a quick table in the README listing all typical envvars / ant options that can be passed to our deployment infrastructure, their meaning and typical usages.
/cc @rleigh-dundee