Reduce Sphinx noise in generated docs

[tamuri]

• Remove the linkcheck command from sphinx-build. Takes a while and most of the URLs are broken or behind paywall. Can be run on demand when someone decides to do something about it.
• Replace the link to source code in Reference docs with a link to Github. This will mean the Sphinx copy of the source code would be redundant, and could also be removed.
• Remove "View page source" links (and, so, remove copy of source)
• Remove "Generated by Sphinx" footer