Re-arrange navigation to fill out 'Administration' topic

[evankanderson]

Related to #6305 and #6412

Proposed Changes

• The previous "install" pages (which included upgrade and uninstall) were moved to Administration in Move install docs to administration #6403, but there were still a lot of "how to install" documents scattered throughout the repo (particularly in eventing)
• Similarly, administration topics for configuring Knative were scattered between "Serving" and "Eventing", and pages which could be done by non-privileged users were mixed with documents that required administrator privileges to run.
  • I had to split the Kafka and RabbitMQ broker docs, but most docs clearly targeted one role or another.
• This PR does not generally move the actual pages (and URLs), but instead restructures the navigation. This somewhat kicks the can of moving things down the road. I chose to do this because:
  1. Given past history, this may need a little time to settle, and moving documents means we need to put in redirects (more toil)
  2. Moving document means updating links, and I'd like to fix a bunch of the internal link warnings in --strict mode, so we can detect when we break these links with higher reliability.

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[evankanderson]

New content starts here: https://deploy-preview-6428--knative.netlify.app/development/administer/

[evankanderson]

/assign @iRaindrop @dprotaso

[knative-prow]

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[iRaindrop]

It's a good restructuring. Good to the Administrator node with content that applies to both Serving and Eventing. Navigation is more intuitive. Important concepts are more prominent.

[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, evankanderson

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[dprotaso]

/cherry-pick release-1.19

[knative-prow-robot]

@dprotaso: once the present PR merges, I will cherry-pick it on top of release-1.19 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.19

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[knative-prow-robot]

@dprotaso: new pull request created: #6430

In response to this:

/cherry-pick release-1.19

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