Add Getting started and User guide to docs

This PR (mainly) adds a getting started guide to the docs, and a more advanced user guide to show the central elements of the psy-view widget.

• Tests passed (for all non-documentation changes)

Note that for #25 (closed), I did add a section to the user guide that recommends not to change the colormap via matplotlibs built-in tool.

closes #38 (closed), closes #37 (closed), closes #25 (closed)