[Fix] Add DataPlane docs to the index (#764)

## Changes
Add DataPlane docs to the index

## Tests


- [ ] `make test` run locally
- [ ] `make fmt` applied
- [ ] relevant integration tests applied

Author: hectorcast-db

docs/index.rst

@@ -17,6 +17,7 @@ We are keen to hear feedback from you on these SDKs. Please `file GitHub issues
    pagination
    logging
    dbutils
+   dataplane
    clients/workspace
    workspace/index
    clients/account