Cluster-api: Docs: evaluate if possible to remove install.md

[fabriziopandini]

Detailed Description
rif. https://github.com/kubernetes-sigs/cluster-api/pull/2590#discussion_r389754110

The Cluster API quick-start imports the install.md file which is basically the quickstart minus setting up the workload cluster. If possible, we should we get rid of this file and merge all its content into quick-start.md.

Anything else you would like to add:
Before implementig the change, it should be verified if the install.md file is used elsewhere.

/kind documentation
/priority backlog

[ncdc]

At today's meeting, we agreed to remove the duplication.
/help

[k8s-ci-robot]

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In response to this:

At today's meeting, we agreed to remove the duplication.
/help

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.

[randomvariable]

Had rewritten a chunk of the quickstart in #2628 so have included this change as well.

/assign
/lifecycle active