Update code insights planning process

Created by: felixfbecker

This is a proposal for how we can mature the planning process we used through FQ2, facilitated by the new GitHub project Beta, so it is easier to repeat and scales to our new team size and product maturity.

It aims to set clearer expectations for when and how in the iterations planning happens than we had previously documented. One of the goals is to be as asynchronous as possible, while still allowing synchronous discussion (in the weekly sync and ad-hoc). As before, all teammates are given agency in planning (planning does not happen top-down). It also includes the expectation for maximum workload and how to deal with overflowing issues to avoid overloading teammates or "crunching" in iterations (we want to work sustainably – that's the main reason we're planning at all). It also adds some notes on how we deal with the mismatch between iterations<->release dates.

Additionally, this PR documents part of our process/philosophy we had been practicing but hadn't documented yet, namely how we approach longer-term (multi-iteration) projects: The process of @Joelkw's product RFCs, the following design/planning phase, and finally the implementation phase – which we parallelize to plan the next project ahead, but constrain to never have too many things going on at the same time.