Docs: Add a sample app that exercises all aspects of Knative

[skonto]

Describe the change you'd like to see
Istio has this nice sample app that demonstrates how to use it.
It would be nice to have a non-trivial example to demonstrate similar concepts in Knative. Not sure if there is any but for example for Eventing I didn't find anything beyond hello-wrold like samples..
I would suggest an app that is aligned with common Knative use cases eg. an IoT app that processes sensor readings using a microservices based approach. It could use a different language per microservice.

/cc @matzew @slinkydeveloper @abrennan89 WDYTH?

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[abrennan89]

@omerbensaadon this one may interest you?

[omerbensaadon]

@abrennan89 totally, this feels like a deliverable that ought to come out of the UX WG and is currently coming to light in User Interviews.

CC: @csantanapr

[omerbensaadon]

Once we wrap up this cycle of interviews (I think next week?) we may want to take a crack at creating an epic and splitting this issue into a few smaller issues to be worked.

[slinkydeveloper]

That's nice!

[skonto]

@omerbensaadon let me know if I can help there.