chg: enable helm-docs and helm schema tools

[piontec]

Summary

• Add helm-values-schema-json pre-commit hook to auto-generate values.schema.json from annotated values.yaml comments.
• Add schemalint pre-commit hooks to normalize and verify the generated JSON schema.
• Add helm-docs pre-commit hook to auto-generate per-chart README.md from values.yaml comments.
• Annotate all attributes in values.yaml with @schema type annotations and # -- helm-docs comments.
• Add .schema.yaml configuration file for the schema generator.
• Add README.md.gotmpl template for helm-docs.
• Update repository README.md with references to the generated chart documentation and schema files.
• Add CHANGELOG entry for the new tooling.

Test plan

• All pre-commit hooks pass (pre-commit run -a).
• dabs.sh -c helm/external-secrets build and lint passes.
• Verify the generated helm/external-secrets/README.md renders correctly.
• Verify values.schema.json validates user values files correctly.

Generated with Claude Code