doc: Minor updates to getting started doc

[fdegir]

This PR makes minor updates listed below to help others deploying Prow.

• Move Commit statuses in repo permissions bullet list to order permissions alphabetically
• Add a heading for prow namespace creation as some of the steps require namespace to exist
• Add link to Prow CRD

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cjwagner, fdegir, krzyzacy

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/prow/blob/main/OWNERS)~~ [cjwagner] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment