add SearchService2 to Linking to External Services documentation and Prezi3 documentation

[mixterj]

To align the Search API to current Prezi 3 and Image 3, we should to make the SearchService2 a known/listed service extension in the Linking to External Services documentation and the Prezi 3 service table (https://iiif.io/api/presentation/3.0/#service)

[azaroth42]

Should define in search2, and list in the registry. No need to change prezi. Removing Presentation label.

[glenrobson]

So this registry: https://iiif.io/api/registry/services/ and annexe doc: https://iiif.io/api/annex/services/ (which needs migrating)

[digitaldogsbody]

Information to be migrated from Annexe docs to Registry docs