0.6rc0 schemas not served from homepage

[jo-mueller]

I just noticed over at the validator of 0.6 data, that the schemas apparently aren't being served correctly from the ngff domain, which makes the validator fail.

This works: https://ngff.openmicroscopy.org/0.5/schemas/image.schema

this doesn't work: https://ngff.openmicroscopy.org/0.6rc0/schemas/image.schema

... and this does: https://ngff.openmicroscopy.org/dev/schemas/image.schema

As it currently stands, there are generally two ways to get the schemas from the web:

• Either by pulling them from content that is served from this page, i.e., by following the above links. When 0.6 is released, there should be another submodule on this page which then provides the schemas under https://ngff.openmicroscopy.org/0.6/schemas/something.schema besides the dev branch for the latest schemas.
• Directly from the release of the ngff-spec repo, i.e., like this: https://raw.githubusercontent.com/ome/ngff-spec/0.6rc0/schemas/image.schema. This doesn't currently work for the older versions, because they are not tagged and released formally from the ngff-spec page.

We could simply create the tags for the past versions, then the second option would also work for all versions.

cc @jni since we talked about this recently.