Publish official Helm chart to a chart repository

[alexluong]

The Helm chart currently lives inside the repo under deployment/kubernetes/charts/ with no published chart repository. Users have to clone the repo, navigate to the charts directory, and run helm package manually.

We should publish the chart to an official Helm chart repository so users can install and upgrade with:

helm repo add hookdeck <repo-url>
helm install outpost hookdeck/outpost

TODO

• Create hookdeck/outpost-helm repo
• Move/copy chart source into outpost-helm
• Set up chart-releaser GitHub Action to auto-package and publish on push/release
• Configure GitHub Pages to serve the chart repo
• Ensure Chart.yaml version is bumped as part of the release process
• Update install docs to reference the new Helm repo