Update knative/docs/install with clear sub-headings for each section

[omerbensaadon]

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

When users hit the /docs/install page, they see a barrage of blue links without a clear understanding of where to go for their needs.

This proposal is to put clear headers on the install page

How will this impact our users?

Users will have a better idea of where to go and won't see a cluster of links close together without any additional context.

Context

This could be grouped in with the "Installing Knative Epic" in docs.

Prototype

[eletonia]

/assign

[omerbensaadon]

knative/docs#3329

Is relevant to this!

[omerbensaadon]

After reviewing the changes in knative/docs#3329 I think they are good enough and solve the original problem of barraging users with links.

After reviewing the comments in knative/docs#3334 (specifically those related to not creating a new "Custom Install" page with a list of .yamls and their dependencies) I think a "Custom Install" page is not exceedingly necessary if we do a good job of explaining the components in the .yaml installation.

I am closing this issue as a result!