docs: Add values.schema.json and NOTES.txt

[lfrancke]

Ports of

• feat: Add values.schema.json hive-operator#743
• docs: Add NOTES.txt and document the values only the schema mentioned hive-operator#753

This will go into each repository. Not templated on purpose.

[dervoeti]

LGTM, but one thing was caught when I had Claude review this:

Helm injects global into every subchart's values before validating them against the subchart's schema. It does this unconditionally, even when the parent chart defines no globals at all, so wrapping this chart in any umbrella chart now fails.

global is blocked in our Chart because of additionalProperties: false at the root level.

Suggested fix:

"global": {
    "title": "Global values",
    "description": "Values shared with parent and sibling charts when this chart is used as a subchart. Not read by this chart.",
    "type": "object"
  }

This sounds like a valid concern to me and I was able to reproduce it. Maybe some customers would like to wrap our charts. Maybe this would affect some ArgoCD deployments as well ("app of apps").

See https://helm.sh/de/docs/chart_template_guide/subcharts_and_globals/

We'd have to fix this in all our values.schema.json, I saw that at least hive-operator has one.

[lfrancke]

Fixed by adding the globals block as suggested. And sorry, I used jujutsu and forgot to create another commit on top. Hence the force push. I'll fold it into the open Hive PR as well.