Feat/add simple vault example

[matzegebbe]

Description

A simple example of using the Sealed Secrets operator (which is deployed directly with). The README also explains how to start a local Vault service for testing and populate it with a secret.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New example (change which adds a new example)

All Submissions:

• A corresponding issue exists
• Have you added a related PR to the kluctl.io repository
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• Have you lint your code locally before submission?
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• All commits are signed off which certify that you created the patch and that you agree to the Developer Certificate of Origin