Add delegation token framework documentation

The intention here is mainly:

• Update the following documentation: https://nightlies.apache.org/flink/flink-docs-master/docs/deployment/security/security-kerberos/
• Create similar readme for Flink just like in Spark because this won't be a super easy to understand feature: https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/security/README.md