Add links to Allocator Service APIs

[jhowcrof]

/kind documentation

What this PR does / Why we need it:
The gRPC API is defined in a proto and the REST API has a swagger.json file. Since the API differs slightly from the GameServerAllocation CRD, these links are pretty helpful for using the service.

Add tip about passing data using metaPatch.

[markmandel]

Assuming all passes, this LGTM. @pooneh-m this is your area, any comments? 😄

[agones-bot]

Build Failed 😱

Build Id: 6130595c-eb11-419f-9736-aa999ac7bcf2

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[markmandel]

--- FAIL: TestGameServerAllocationPreferredSelection (381.00s)
    gameserverallocation_test.go:379: error waiting for fleet condition on fleet preferred-2bjwj
FAIL

That's a new flake. Retrying.

[agones-bot]

Build Succeeded 👏

Build Id: 85abae21-0114-4375-b12e-f0d1b7fb4ef3

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

• image: gcr.io/agones-images/agones-controller:1.13.0-23225a4
• image: gcr.io/agones-images/agones-sdk:1.13.0-23225a4
• image: gcr.io/agones-images/agones-ping:1.13.0-23225a4
• Linux C++ SDK (build): agonessdk-1.13.0-23225a4-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.13.0-23225a4.zip

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

• https://23225a4-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2023/head:pr_2023 && git checkout pr_2023
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.13.0-23225a4

[pooneh-m]

thanks for updating the documentation.

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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