Add sanity.io to Hosting

[regisrob]

https://www.sanity.io/docs/iiif-api-reference

But sample links in this page does not work anymore...

This also raises the question of having two lists of hosting services to maintain (awesome-iiif + iiif.io https://iiif.io/get-started/vendors/#hosting-services), which are currently different.

[regisrob]

The same remark can be made for the CMSs/DAMs list: https://iiif.io/get-started/vendors/#digital-asset-management-dams-collections-management-and-repository-tools-that-support-iiif

[triplingual]

Could be considered in parallel to #403