New Release Process (apache#301)

* Copy release process doc from Apache Airflow

Source: https://github.com/apache/airflow/blob/fb741fd87254e235f99d7d67e558dafad601f253/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 <[email protected]>

* Add sign.sh script for release artifacts

Script is copied from https://github.com/apache/airflow/blob/395ad7110e53a30a5d33f648d1dd797482eb268c/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 <[email protected]>

Relates to apache#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.

README.md

@@ -260,13 +260,4 @@ tips and tricks for troubleshooting common issues. Please examine these first be
 
 ## Release Process
 
-1. Bump the version in [charts/pulsar/Chart.yaml](https://github.com/apache/pulsar-helm-chart/blob/master/charts/pulsar/Chart.yaml#L24).
-
-2. Send a pull request for reviews.
-
-3. After the pull request is approved, merge it. The release workflow will be triggered automatically.
-   - It creates a tag named `pulsar-<version>`.
-   - Published the packaged helm chart to Github releases.
-   - Update the `charts/index.yaml` in Pulsar website.
-
-4. Trigger the Pulsar website build to make the release available under https://pulsar.apache.org/charts.
+See [RELEASE.md](RELEASE.md)