Remove Helm charts, which have moved to a separate repo

[0xThresh]

Pull Request Checklist

• Description: Briefly describe the changes in this pull request.
• Changelog: Ensure a changelog entry following the format of Keep a Changelog is added at the bottom of the PR description.
• Documentation: Have you updated relevant documentation Open WebUI Docs, or other documentation sources?
• Dependencies: Are there any new dependencies? Have you updated the dependency versions in the documentation?
• Testing: Have you written and run sufficient tests for the changes?
• Code Review: Have you self-reviewed your code and addressed any coding standard issues?

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Description

This PR removes the Helm templates and adds a README in their old location to point users to the new repo, https://github.com/open-webui/helm-charts.

Changelog Entry

Added

• Adds kubernetes/helm/README.md

Fixed

• N/A

Changed

• N/A

Removed

• All Helm files, including templates and chart, under the kubernetes/helm folder

Security

• N/A

Breaking Changes

• N/A

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Additional Information

• Closes feat: Publish helm chart to GitHub Helm Registry helm-charts#23. Justification for moving Helm charts to a separate repo can also be found there.

[tjbck]

LGTM, Thanks!