docs(dgraph4j): simplify Java client docs and update for v25

[mlwelles]

Summary

• Simplifies the Java client docs (~528 lines → ~102 lines) to match the concise format used by the Go and Python client docs
• Adds v25 APIs: connection strings (DgraphClient.open()), runDQL(), setSchema()
• Links to the dgraph4j GitHub README for complete API documentation
• Updates supported versions table to show 23/24/25

Blocked

Do not merge until dgraph-io/dgraph4j#284 (v25 upgrade) is merged, as the docs reference v25 APIs (DgraphClient.open(), runDQL(), setSchema(), etc.) that only exist on that branch.