docs: update documentation contribution guidelines

[markmandel]

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
/kind release

What this PR does / Why we need it:

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

I've repeated these details a lot over the years, I should update the documentation.

[agones-bot]

Build Succeeded 🥳

Build Id: 3c70dee6-a187-4836-b9c5-a7794f353653

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:1.58.0-dev-efe3655
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.58.0-dev-efe3655
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.58.0-dev-efe3655-linux
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.58.0-dev-efe3655
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.58.0-dev-efe3655
• image: us-docker.pkg.dev/agones-images/ci/agones-processor:1.58.0-dev-efe3655
• Linux C++ SDK (build): agonessdk-1.58.0-dev-efe3655-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.58.0-dev-efe3655.zip

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

• https://efe3655-dot-preview-dot-agones-images.appspot.com/

To install this version:

git fetch https://github.com/googleforgames/agones.git pull/4534/head:pr_4534 && git checkout pr_4534
helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.58.0-dev-efe3655