Update developer documentation / policy to enforce green builds

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