Tamland documentation: a day-in-the-life of Capacity planning

@abrandl Kindly offered in Slack to:

explain "a day in capacity planning" to try to give an idea how we think about it at the moment and from .com experience.

The context was questions around tuning (or not tuning) tamland, but can extend to how to do good capacity planning in general. The goal is to provide something that Dedicated SREs (who are not Scalability or Tamland experts) with some useful guide-rails and hints on how to effectively perform Capacity planning using the relatively new process to them.