feat(helm): Improve docs, add schema

[kiblik]

• README in the form of a symlink was dropped. It was generating some ugly warnings until now
• readme-docs/KUBERNETES.md was replaced with a simple description where the information can be found
• No content from README.md/KUBERNETES.md was removed, only
  • moved to another location (README.md.gotmpl)
  • extended by autogenerated help (list of available variables)
• add hint for string/nil values
• add values.schema.json - this is useful for values validation before manifests are sent to the k8s API
• add native linter
• add generator/validator of documentation (if you updated values.yaml but did not mention it in the documentation, GHA will fail)

There is still a huge place for improvement (render comments from values.yaml to descriptions, add $ref for better validation of k8s resources, mention all available variables, add no-additional-properties, ...). Still, it is a good second step (the first was definitely #12691) for improving the helm chart.

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[kiblik]

@fernandezcuesta, I was planning to autogenerate values.schema.json + documentation using

• https://github.com/losisin/helm-values-schema-json
• https://github.com/norwoodj/helm-docs

However, I bumped into the following issue:

- at '/postgresServer': got null, want string
- at '/imagePullSecrets': got null, want string
- at '/redisServer': got null, want string

I know that I would be able to forcefully patch related places with type: [null, string], but it does not look/sound good.
Do you have any recommendations for a better way to generate schema and docs automatically? I wasn't able to fix it with any helper/annotation in the values.yaml file using the mentioned tools.

[fernandezcuesta]

Hi, I guess it's gonna be easier to set those 3 to an empty string rather than nil also to be consistent with the rest of default values.
On the other hand, it's weird that annotations aren't doing the job (it should be pretty much the same as this example)

[kiblik]

Hi, I guess it's gonna be easier to set those 3 to an empty string rather than nil also to be consistent with the rest of default values. On the other hand, it's weird that annotations aren't doing the job (it should be pretty much the same as this example)

Ok, it looks like I made so silly typo when I was testing it before because now it works perfectly.

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