[EPIC] Create "Securing Kubernetes and Knative" section for admin guide

**Describe the change you'd like to see**

We should document in the admin guide how to set up Knative and Kubernetes securely (referencing up-stream guides as appropriate).

Topics might include (this is an initial brainstorm, and we should think carefully about which bits belong in our docs vs being better as references to upstream docs):

- Link to our threat model, to be clear about currently supported/unsupported use cases
- Link to our vulnerability reporting processes
- Namespace isolation considerations (e.g. suggested network policy)
- Protecting shared components
- Securing pods
- "Out of the box" protections (e.g. minimal set of allowed volumes, subset of securityContext etc)
- Description of relevant feature flags
- Using PodSecurityPolicies and custom runtimes (e.g. Kata, gVisor)
- Private registries

**Additional context**
Add any other context or screenshots about the feature request here.

/assign @evankanderson for thoughts
/assign @RichardJJG is there a template we should start from for this?