Add an "About Knative" page

[snneji]

Task

1. Add a README.md page to the "About" section that includes the following conceptual info:
   • high level info about Knative Serving + Eventing
   • architecture info
   • diagrams
2. The page should be similar to Istio: https://istio.io/latest/about/service-mesh/
3. The page should be the first page in the "About" section as it will be like an overview page.
4. Page must have links to Serving, Eventing, Install
5. Replace the link for the "Explore Knative" button on the homepage with a link to this page.

Questions to answer as follow up from #4668

6. What is Knative?

Knative is a system that works on top of Kubernetes. It helps development teams manage, monitor, and operate Kubernetes in a way that requires less technical knowledge and time. Knative is open-source, finding support and growth through a combination of individual contributors and industry-leading companies.

7. What's the difference between Knative Serving and Knative Eventing?

Knative has two main elements named Knative Serving and Knative Eventing. While they manage different tasks and outcomes and can operate independently, together, they help make Knative a powerful tool for dev teams and IT professionals.
Knative Serving runs serverless containers on Kubernetes with ease. Knative takes care of the details of networking, autoscaling (even to zero), and revision tracking. You have to focus on your core logic. Knative Eventing allows for universal subscription, delivery, and management of events. Teams can build modern apps by attaching compute to a data stream with declarative event connectivity and a developer-friendly object model.

8. Do I need to use Istio?
9. What foundation is it under (CNCF, LF, etc)?
10. Does Knative support serverless functions?
11. Link to spin off section about users / adopters / case studies

Additional info

Spin off section about users / adopters / case studies
(TODO: decide on name for this)

• Testimonials
• Case Studies
  • deepc
  • outfit7
  • puppet
  • NYT (future)
  • Optum (future)
• Is there a managed cloud version of Knative?
• Who are the contributors or companies backing the project?
• Who is using Knative ? -> Link to Adopters

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[abrennan89]

As far as I'm concerned, this page could also cover all the requests proposed on #4668 and we can close that one?

[abrennan89]

Removed FAQ from the list because I think these are sort of a nonsense thing and shouldn't be required if our info is laid out properly so that it's easy to find.

[abrennan89]

I'd say case studies and testimonals should also be a separate page, not part of "About Knative" since they're not really explaining anything conceptual.