Add currently used mu-plugins to docs

All of these are technically in various stages of progress, but implemented, so I'm including them in one primary section.

Once a production release has occured, it may be worth adding an in progress section for mu-plugins. Though - these will likely be developed as optional plugins before making the conversion to must use.
Use 'Available Global Plugins' rather than 'Available Plugins'

Trying to be consistent with terminology here. Plugins are available on the global WSUWP platform and can be activated for networks and sites.