Tidying script docs & using git links (rebased onto dev_4_4)


This is the same as gh-558 but rebased onto dev_4_4.

Note: contributing docs are staying in the OMERO developer section on this branch so those links have not been updated. However, if using-git.txt is significantly reworked while we are still pointing at version 4 docs by default, I will update the links to point at the new contributing docs.

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Adds the missing screenshot of the user scripts in the client menu, the Python for biologists help link that @jburel emailed me about, and tidies the links to the new contributing developer docs.