Admin and Install docs

[iRaindrop]

Write an Aministration Overview and finish up high-level remaining tasks for install docs.

Proposed Changes

• Add Administration Overview to the doc set. Summarize task areas for a Knative admin with links.
• Possibly move config content from install topics to the admin node in the repo.
• Update the Installation Roadmap for admin install context.
• Update quickstart install docs.

[knative-prow]

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