Reformatting VA docs to make them less scary  (rebased onto develop)



This is the same as gh-847 but rebased onto develop.

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See https://trello.com/c/AmB1db6W/57-5-0-3-documentation-fixes 
@kennethgillen commented that the VM docs are rather intimidating - not that all the info isn't useful, but being faced with this epic file all in one go makes it seem far more complicated than it is. So I've split the content up. Also some tidying for whitespace & updated help link.

See https://www.openmicroscopy.org/site/support/omero5-staging/users/virtual-appliance.html for review.