Adding OMERO version history to docs (rebased onto dev_5_0)



This is the same as gh-949 but rebased onto dev_5_0.

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See https://trello.com/c/lxe4JUu8/73-move-omero-version-history-from-plone-to-sphinx-docs

The plan agreed with Jason is for this content to be duplicated from plone and then replace it once the current funding round is over (Nov or thereabouts). Version history in the docs makes OMERO on a par with BF - both docs pages can then be linked from the website and we have one less place to update on release day rather than in advance.