Add a diagram for knative architecture about docs

[abrennan89]

@bbrowning @markusthoemmes would you be able to provide this or know who could?

[knative-housekeeping-robot]

Issues go stale after 90 days of inactivity.
Mark the issue as fresh by adding the comment /remove-lifecycle stale.
Stale issues rot after an additional 30 days of inactivity and eventually close.
If this issue is safe to close now please do so by adding the comment /close.

Send feedback to Knative Productivity Slack channel or file an issue in knative/test-infra.

/lifecycle stale

[knative-housekeeping-robot]

Stale issues rot after 30 days of inactivity.
Mark the issue as fresh by adding the comment /remove-lifecycle rotten.
Rotten issues close after an additional 30 days of inactivity.
If this issue is safe to close now please do so by adding the comment /close.

Send feedback to Knative Productivity Slack channel or file an issue in knative/test-infra.

/lifecycle rotten

[samodell]

/remove-lifecycle rotten

[abrennan89]

Covered in part by: knative/eventing#2221 cc @aslom

Closing since this information will likely be crowdsourced from technical teams then added to docs in parts under multiple PRs; no one docs person responsible for creation or maintenance of an overall diagram.