Add a "why" section for the Allocator Service documentation

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, hit 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:

We get questions about which tool to use when allocating a GameServer - so here are some docs to help answer that question!

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

N/A

[markmandel]

@pooneh-m would particularly like your input here, see if I missed anything.

[agones-bot]

Build Failed 😱

Build Id: 959c2d60-3847-4ad4-b670-b0678c89ff1d

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[agones-bot]

Build Succeeded 👏

Build Id: ae68374e-94e4-4dc0-82b8-5f42cf9b1af8

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

• image: gcr.io/agones-images/agones-controller:1.12.0-11865aa
• image: gcr.io/agones-images/agones-sdk:1.12.0-11865aa
• image: gcr.io/agones-images/agones-ping:1.12.0-11865aa
• Linux C++ SDK (build): agonessdk-1.12.0-11865aa-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.12.0-11865aa.zip

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

• https://11865aa-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1953/head:pr_1953 && git checkout pr_1953
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.12.0-11865aa

[pooneh-m]

Thanks for the addition! It is useful.

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, pooneh-m

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[google-oss-robot]

New changes are detected. LGTM label has been removed.

[agones-bot]

Build Failed 😱

Build Id: 28369a46-64ce-4b5d-807e-5301b817c27d

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[agones-bot]

Build Succeeded 👏

Build Id: 51e98088-2254-4b92-989f-45b90cc86454

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

• image: gcr.io/agones-images/agones-controller:1.12.0-c3f5217
• image: gcr.io/agones-images/agones-sdk:1.12.0-c3f5217
• image: gcr.io/agones-images/agones-ping:1.12.0-c3f5217
• Linux C++ SDK (build): agonessdk-1.12.0-c3f5217-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.12.0-c3f5217.zip

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

• https://c3f5217-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1953/head:pr_1953 && git checkout pr_1953
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.12.0-c3f5217