Add an example of setting autoscaler behavior in a Fleet.

[roberthbailey]

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug

/kind cleanup

/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it: Clarifies the documentation added in #2754 by showing an example using a Fleet in addition to a GameServer.

Which issue(s) this PR fixes:

Part of #2747

Special notes for your reviewer:
n/a

[agones-bot]

Build Succeeded 👏

Build Id: f81a6d99-92bb-43cd-ac3a-c238ab20030e

The following development artifacts have been built, and will exist for the next 30 days:

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.27.0-3e1d2a1-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.27.0-3e1d2a1-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.27.0-3e1d2a1-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.27.0-3e1d2a1-amd64
• Linux C++ SDK (build): agonessdk-1.27.0-3e1d2a1-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.27.0-3e1d2a1-amd64.zip

A preview of the website (the last 30 builds are retained):

• https://3e1d2a1-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2759/head:pr_2759 && git checkout pr_2759
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.tag=1.27.0-3e1d2a1-amd64

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, roberthbailey

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• OWNERS [markmandel,roberthbailey]

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