Generate values.schema.json

[NotMyFault]

What does this PR do?

Starting with helm 3, you can provide a schema for your values.
The change proposed adds an action to do so upon PRs or request.

• Fixes #

If you modified files in the ./charts/jenkins/ directory, please also include the following:

### Submitter checklist
- [x] I bumped the "version" key in `./charts/jenkins/Chart.yaml`.
- [x] I added a new changelog entry to `./charts/jenkins/CHANGELOG.md`.
- [x] I followed the [technical requirements](https://github.com/jenkinsci/helm-charts/blob/main/CONTRIBUTING.md#technical-requirements).
- [x] I ran `.github/helm-docs.sh` from the project root.

Special notes for your reviewer

[timja]

I assume this fails the build if its out of date?

What's the expected workflow?

If a developer is supposed to run something then:

1. Needs update to https://github.com/jenkinsci/helm-charts/blob/main/CONTRIBUTING.md
2. Update to PR template