Add documentation and extend the XAS modes

[mretegan]

The documentation built from the current merge request: https://hdf5.gitlab-pages.esrf.fr/nexus/nxxas/classes/applications/NXxas_new.html

An editable document for the working group can be found here https://docs.google.com/document/d/1M-9ViaEiaR6fUwb5IffQILodOSTvJq5s.

The contents of that document will be put in the doctring.

[newville]

sorry for the delay -- I'll merge this, then make a few modifications

[mretegan]

@newville I think @maurov updated the Google document, but I did not have time to add that info to the NeXus definition. I should have marked this as a draft, as it was not ready to merge.