doc: Add documentation on how to directly interact with the schema registry

The schema registry [doc](https://conduit.io/docs/features/schema-support/) says
> work directly with the schema registry (for example, in cases where automatic schema extraction isn't enough and a schema needs to be built manually using the information from a source)

But I dont really see much information on how to do that anywhere.

I see that there's a [REST API](https://github.com/ConduitIO/conduit-schema-registry?tab=readme-ov-file#rest-api) for the Schema Registry

There is a schema.go package in the [connector sdk ](https://github.com/ConduitIO/conduit-connector-sdk/blob/main/schema/schema.go) and [processor sdk](https://github.com/ConduitIO/conduit-processor-sdk/blob/main/schema/schema.go). Is that we should use? It seems to have fewer functions than the REST API...

And the schema registry seems to use the [franz sr](https://github.com/twmb/franz-go/tree/master/pkg/sr) package under the hood. Should that ever be used?

Some more docs on how to use it and especially with some real-world examples/use cases would be very helpful.

Start with the schema-support documentation, the schema registry REST API README, and the connector and processor SDK schema.go files mentioned in the issue. Compare those interfaces with the franz-go sr package, then document the recommended approach with real-world examples; done means users can understand how to interact with the registry directly and which API to choose.

Written by the indexing model from the issue text.