Rename 'Usage' header to 'Features'

Refs #12279
Moved intro documentation on cluster setup.

Moved documentation in 'usage/cluster' related to initial setup of
creating vms and importing orphans to 'importing'. Removed duplicate
information and added warning about read-only cluster mode.

Refs #12279
Move upgrading documentation under usage docs.

Created new header in index for Usage which is seperate from features.

Refs #12279
Added a lot of placeholder docs with TODOs.

This includes quite a bit of rearranging on the index page.

Added Usage section back, so there now exists both Usage and Feature
documentation.

conf.py now contains the default setting for todo_include_todos. This
can be set to True to generate documentation that displays todos. It
will not be picked up by sphinx-build though, so the build directory
will have to be rm-ed first.

Refs #12279
Bump release version in Documentation.

Refs #12279
Fix up README headers.

Refs #12279
Update README to RST format.

Reformatted the README to be structured in rst. This way it should
actually render correctly on Github.

Refs #12279