Update schemas doc pages

[joshmoore]

The landing pages:

• top-level: https://www.openmicroscopy.org/Schemas/
• per-spec: https://www.openmicroscopy.org/Schemas/OME/
• per-version: https://www.openmicroscopy.org/Schemas/OME/2016-06/

could provide a little more guidance to the various representations
of the schema, especially delineating between the XSD file and the
generated docs. This PR will collect minor updates that will hopefully
not interfere with the update semantics from ome/ome-model.

[sbesson]

Note these pages are auto-generated via the publish script, in the case of these changes, specifically https://github.com/ome/ome-model/blob/908c6ef501f18b329d46a65fb7bd563f0d5580a6/specification/publish#L232-L240. So in the current state, these changes would need to be backported

This obviously raises the question of whether the logic generating these HTML pages should be migrated to this repository instead to simplify the review process

[joshmoore]

Also likely explains the reason that little has been invested in their look-n-feel ;)

Ok. Leaving this here for the moment, but I could certainly see pulling the HTML portion out into the modern markdown world.