Deleting contributing docs on 4.4

We were going to leave these docs on 4.4 but now the jobs have all changed, the CI doc is all broken anyway so I decided it was best to just delete the lot. The only page within this section that is referenced by the rest of the docs (outside the contributing section) is the using-git page so I've added 2 hard-coded links to that to save :doc: links across branches, and just put in a note to redirect people on the front page.

This should fix all the broken links on the OMERO-docs-merge-stable build except the ones in the C++ bindings page.