Web app docs 11730 dev 4 4

This PR documents 2 main changes (see https://trac.openmicroscopy.org.uk/ome/ticket/11730)

--rebased-from #581

Same features are in dev_4_4 as develop:
Django apps can be anywhere on PYTHONPATH openmicroscopy/openmicroscopy#1813
App settings openmicroscopy/openmicroscopy#1789

This PR 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.