Page documenting Serving feature flags

[mattmoor]

This isn't itself new, but the current docs only address the meta part (what's a feature flag), but not what flags exist today.

Kind of what I was thinking was something along the lines of https://knative.dev/v0.16-docs/serving/autoscaling/kpa-specific/

However, covering the sorts of feature flags from: knative/serving#9072

I'd love to have a place to have folks document new flags like the above with a relatively quick edit, but we need to bootstrap that page first.

cc @dprotaso @JRBANCEL @abrennan89

[JRBANCEL]

/assign