Add sitemap, provenance, dataset markup

[IgorRodchenkov]

See, follow:

• https://ai.googleblog.com/2017/01/facilitating-discovery-of-public.html
• https://developers.google.com/search/docs/data-types/dataset
• https://schema.org, specifically https://schema.org/Dataset

One of our datasets would be the set of files in the downloads page.

The pc2/datasources page could have isBasedOn property for each listed data provider.

We could even generate and add to the sitemap app-ui query URL for each gene name...

I believe, all such markup, metadata content could be configured and simply injected into pages with Nginx (i.e., we do not necessarily have to do this for each our app, site, etc..)