Document 2-week iterations for the web team

Created by: felixfbecker

As decided in our 3.20 retrospective we want to try 2-week iterations on the web team, to only have a single goal per iteration and everyone working on the same project per iteration, avoiding costly context switches within the iteration. Our first iteration starts next Monday (September 28). We also want to ship smaller PRs and use smaller individual issues. To accomodate the faster cycles and communicate our plans and progress better within the team, we're going to use a GitHub project board for each iteration instead of a tracking issue (we may still create tracking issues for projects though). The prose goal description from the tracking issue is moved to a living Google doc. We're also therefor dropping the weekly teammate updates on those, as we now have the weekly update for stakeholders on the mailing list and the daily update in Slack for ourselves. Retrospectives are helt after each iteration instead of monthly.

I expect we'll refine this process more and add more details to the handbook as we figure things out to keep it in sync.