Update developer documentation / policy to enforce green builds

[fjetter]

Our recent CI issues is causing alert fatigue and we're used to merging PRs without positive confirmation from our CI builds. We should not do this.

I propose to update our developer documentation / policy (do we have any formal policy?) to not allow merging of red PRs. If flaky issues are known, they should be marked as such and have a corresponding tracking issue available

Blocked by

• Getting CI green again
• Cleaning up our issue tracker from stale flaky test issues

[gjoseph92]

Changing GitHub settings so people with merge access don't have permissions to override failing CI would be an effective if heavy-handed way of enforcing this.