Implement automated release pipeline

[timflannagan]

Introduce GHAs to automate the release process.

DoD:

• Workflow triggered on PRs and runs a subset of the release stages to validate release pipeline consistently works
• Merges to main publish :latest, :main, :nightly (or some variation of that) to ghcr.io
• Package and publish the kgateway helm chart
• Validation job that runs on non-PR events to validate the release is pullable, functional, etc.
• Add quickstart instructions for how to consume the release
• Documentation so devs know how the cut a release
• All relevant images (controller, sds, envoy-init) are published under the correct registry and naming convention

Partially blocked on <TODO: find issue for release cadence>.

Out-of-scope:

• Changelogs and release notes
• Any registry management (e.g. periodic blog purging) work items