docs: add DCO sign-off instructions to contributing guide

[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:

Adds a Developer Certificate of Origin (DCO) subsection under the "Submitting code via Pull Requests" section, covering how to sign commits and how to retroactively sign off on existing commits.

Which issue(s) this PR fixes:

Work on #4421

Special notes for your reviewer:

Wrote this while about to get on a plane, so hopefully not too much of a rush 😁

[igooch]

Any reason do choosing this option over the EasyCLA?

[markmandel]

It's not either or - DCO is required. If you also want to add a CLA you can (but it doesn't seem necessary, the less lawyers the better ☺️)

Ensure that DCO are enabled for all GitHub repositories of the project. You may also choose to use a CLA.

[agones-bot]

Build Succeeded 🥳

Build Id: 57a52885-849a-4594-89a8-c5a6bf79782e

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:1.57.0-dev-7da53a0
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.57.0-dev-7da53a0
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.57.0-dev-7da53a0-linux
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.57.0-dev-7da53a0
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.57.0-dev-7da53a0
• image: us-docker.pkg.dev/agones-images/ci/agones-processor:1.57.0-dev-7da53a0
• Linux C++ SDK (build): agonessdk-1.57.0-dev-7da53a0-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.57.0-dev-7da53a0.zip

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

• https://7da53a0-dot-preview-dot-agones-images.appspot.com/

To install this version:

git fetch https://github.com/googleforgames/agones.git pull/4477/head:pr_4477 && git checkout pr_4477
helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.57.0-dev-7da53a0