Improve help messages

[dominikl]

It looks like omero render info actually creates rendering settings in case there aren't any. And omero render test --thumb creates the thumbnails in case there aren't any. But this isn't stated anywhere in the help messages.

[will-moore]

I would also like to see the contents of the help message in the docs on the README, so that you can read them without having to run the plugin.

[jburel]

@will-moore adding that comment to my doc review

[jburel]

some doc pages have followed that approach but not all e.g. https://docs.openmicroscopy.org/users/cli/import.html

[joshmoore]

It requires some twists & turns with the build order, but at least in sphinx, I've had success with https://pypi.org/project/sphinxcontrib-programoutput/