Add GATEKEEPER.md for a guide on enforcing use of Kata Containers

[jensfr]

- What I did
This commit adds GATEKEEPER.md to the docs directory and example yaml
manifests to config/sample/gatekeeper. The document
provides a step-by-step guide on using OpenShift Gatekeeper to enforce
policies that require privileged pods to use Kata Containers.

- Description of the problem which is fixed/What is the use case
Reasons to include this in the repository:

Kata Containers enhance isolation for sensitive workloads. This guide
helps users implement and enforce their use through Gatekeeper policies.
By including this, we aim to:

1. Educate users with clear instructions on enforcing security policies.
2. Promote best practices for securing Kubernetes environments.
3. Improve usability by offering a structured, practical example.

Structure

1. Constraint Template: Steps to create and apply.
2. Constraint: Instructions to create and apply.
3. Pod Manifest: Example for testing.
4. Compliance*: How to verify.

This document enhances the repository by providing practical guidance on
using Kata Containers with Gatekeeper.

- How to verify it
Read it and try it out.

Signed-off-by: Jens Freimann [email protected]

[openshift-ci]

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

[bpradipt]

Should this also apply to higher level objects like Deployments etc, or creating the policy for Pod automatically applies it to higher level constructs that uses Pod (eg. Deployments, Replicasets etc) ?