add release channels and support levels

[astoycos]

Add stable and experimental release channels and Core, Extended, Implementation-specific support levels in line with the gateway-api.

Currently all of the apis we provide are experimental since they are v1alpha1 version.

Also add documentation on the new channels + Support levels and update our NPEP process slightly.

[k8s-ci-robot]

Skipping CI for Draft Pull Request. If you want CI signal for your change, please convert it to an actual PR. You can still manually trigger a test run with /test all

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[astoycos]

/hold

(to prevent accidental merge)

[k8s-ci-robot]

@tssurya: GitHub didn't allow me to request PR reviews from the following users: of, this, to, fill, section, do, a, PR, top, will, on.

Note that only kubernetes-sigs members and repo collaborators can review this PR, and authors cannot review their own PRs.

In response to [this](https://github.com/kubernetes-sigs/network-policy-api/pull/131#discussion_r1294850375): >/cc @tssurya will do a PR on top of this to fill this section >/assign @tssurya Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[k8s-ci-robot]

PR needs rebase.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: astoycos, Dyanngg, tssurya

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/network-policy-api/blob/main/OWNERS)~~ [Dyanngg,astoycos] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[tssurya]

/lgtm