Data Model & OME-TIFF Docs rework

This PR addresses tickets 10695, 11278 and 11279 which are all linked under https://trac.openmicroscopy.org.uk/ome/ticket/10274

It aims to
- clarify and improve the main index page so people can find what info we have more quickly
- make a more approachable OME-TIFF landing page (ome-tiff/index)
- update the list of OME-TIFF supporters and make it easily accessible
- add the XML validation method (currently explained in FAQs only) and temporarily remove reference to the online validator which is not currently working