Build: Replace godoc with pkgsite

[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:

Updating the developer tooling to use pkgsite rather than godoc (which isn't installed any more either - so the command fails).

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

N/A

[agones-bot]

Build Succeeded 👏

Build Id: 6be0c6a2-614e-4f37-a91d-081d3a03e732

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:1.39.0-dev-b34e162-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.39.0-dev-b34e162-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.39.0-dev-b34e162-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.39.0-dev-b34e162-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.39.0-dev-b34e162-amd64
• Linux C++ SDK (build): agonessdk-1.39.0-dev-b34e162-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.39.0-dev-b34e162-amd64.zip

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

• https://b34e162-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3643/head:pr_3643 && git checkout pr_3643
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.39.0-dev-b34e162-amd64