Doc running App from anywhere & CreateApp refactor

This PR documents 2 main changes (see https://trac.openmicroscopy.org.uk/ome/ticket/11730)

 - Django apps can be anywhere on PYTHONPATH https://github.com/openmicroscopy/openmicroscopy/pull/1770
 - App settings https://github.com/openmicroscopy/openmicroscopy/pull/1788

And also refactors the CreateApp page to move docs that are not App-specific to a new page.
If creating an app, you shouldn't really need to run OmeroWeb from the source code. Moving this to a new page makes the CreateApp page simpler - but still provides the info for those who need it. The new EditingOmeroWeb page is linked from the Web intro page and the CreateApp page.