Deploy client-side registration docs not up-to-date for new backoffice

[krebil]

The docs does not contain an example for the new backoffice, the example posted for v15 is angular js.

https://docs.umbraco.com/umbraco-deploy/extending/extending#client-side-registration

[ronaldbarendse]

This part of the documentation does indeed need to be revised, both the server-side C# parts (service connectors only have async methods now and entity type registration has been simplified) and the client-side (using manifests).