Add description and links to DataHub Template schemas

[rabuono]

The instance admin is able to upload Templates to the instance using the feature based on this issue:
seek4science/seek#1062

The Templates follow the schemas described on this issue:
seek4science/seek#1064

It would be nice if there was documentation for instance admins to know where the schemas are.

[floradanna]

It should be added to the main SEEK doc @kdp-cloud

[kdp-cloud]

Is this enough?

[floradanna]

I think so. I would add the link to the specifications if the JSON is up to date. In case, we can add it later