Update documentation

[Kayvv]

No description provided.

[rchristie]

Great work.
Field Editor needs to be updated for Hugh's latest changes to field editor, and that code also needs some fixes!
I get warning on build html:
~\sceneviewereditorwidget.rst: WARNING: document isn't included in any toctree
Some additional changes:

• scene-editor-widget.png - cmiss_number is not really suitable for a Subgroup field. I suggest using the field editor to create a constant field "left ventricle" with value 1 and choosing that for your graphics.
• field-editor-widget.png - need to remake image as it is missing the Field type section. Also, please don't use coordinates_cmiss_rc_wrapper as the selected example as it's an internal auto-generated field. If using the deforming heart example, I suggest using E2 as a good example.