Add "Call to Action" at the end of the Installation documentation

[omerbensaadon]

What is changing? (Please include as many details as possible.)

Include a link to join the Knative Slack or ask a question on Stack Overflow with a standardized "Call to Action" component on the site wherever we anticipate a user is finished installing.

This includes pages like https://knative.dev/docs/install/any-kubernetes-cluster/ and anywhere else we are classifying as an installation page

How will this impact our users?

This will give users a clear "Next Step" once they have finished installing Knative to join the community.

Context

Alongside exit-rates, adding these component and tracking clicks on each will help us better understand how many folks are reaching the end of our installation documentation.

In order to track anything, we must decide what we want users to do once they have installed Knative; Slack and StackOverflow feel like good places to point folks.

The UX WG is open to other opinions as to where we ought to point users at the end of install.

Prototypes

[julz]

Kind of silly idea, but thinking out loud: we do already have a "Feedback: Was this page helpful?" button at the bottom of pages, but I wonder if we could have a more specific action here. e.g. "Was this install painless? [Yes, tell us in Slack, we'd love to hear how you're using Knative] / [No, please follow this link for help options]"

[knative-prow-robot]

@omerbensaadon: The label(s) /label good-first-issue cannot be applied. These labels are supported: proposal/0.11, proposal/0.12, proposal/0.13, proposal/0.14, proposal/0.15, proposal/0.16, proposal/0.17, proposal/0.18, proposal/0.19, channel/kafka, channel/natss, source/awssqs, source/camel, source/ceph, source/couchdb, source/github, source/kafka, source/prometheus

In response to this:

/label good-first-issue

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[Cali0707]

Moving this to ready to work as part of website re-design

[aliok]

We already have a link to Slack here: https://knative.dev/docs/getting-started/next-steps/

[asr2003]

@aliok @Cali0707 @Leo6Leo After the discussions gone through in this issue, can the Stackoverflow CTA can be added? If not, the issue can be closed as of satisfying of Slack CTA exists in docs.