[Documentation]: Upgrading for a newly release Apache Spark version

When a new version of Apache Spark is released, we'd have to make changes in the code base to add any new APIs, account for new protocol changes on the worker-side etc. We should document this to allow anyone to upgrade .NET for Apache Spark to newer versions.

**Success Criteria**
- Clearly call out code changes with examples
- Capture caveats one might encounter during this upgrade process