* Copy release process doc from Apache Airflow
Source: fb741fd872/dev/README_RELEASE_HELM_CHART.md
* Adapt to Apache Pulsar
* Remove old release process notes
* Fix typo
* Apply suggestions from code review
Co-authored-by: tison <wander4096@gmail.com>
* Add sign.sh script for release artifacts
Script is copied from 395ad7110e/dev/sign.sh
* Add some updates (more to might follow)
* Add some more updates to the rest of the release plan
* Fix rat check command
Co-authored-by: tison <wander4096@gmail.com>
Relates to #290
### Motivation
Make the Apache Pulsar Helm Chart release follow ASF rules for voting, and make the helm binary available via dist.apache.org. By following the information in https://issues.apache.org/jira/browse/LEGAL-573 and in the Apache Airflow project https://github.com/apache/airflow/blob/main/dev/README_RELEASE_HELM_CHART.md, I built this new release process. It will likely need some iterative improvement.
### Modifications
* Add a release process that is based on the Apache Airflow release process
### Verifying this change
- [ ] Make sure that the change passes the CI checks.
