Document IIIF 3 support

https://redmine.teklia.com/issues/10642

Requires backend#1972 (closed), backend#1973 (closed), frontend#1611 (closed)

• Update the image docs to mention that both IIIF Image 2.1 and 3.0 are supported

• Update the image server docs:

  • Under Automatic detection, it should be mentioned that the version of the server will be determined by the version of the image that we are creating a server for
  • A new subheading under Manual creation should explain the IIIF version attribute

• Update the server management page to mention that merging requires both servers to have the same version

• Update the export structure to add the new export version number and the version on image servers