Document a few more how-tos Marlin vs k4FWCore

There are a few questions that seem to be general enough to warrant their own section in the documentation for people coming from Marlin:

- #192
- https://github.com/key4hep/k4FWCore/issues/229

Start by reading issue #192 and k4FWCore issue #229 to identify which questions are general enough for newcomers coming from Marlin. Add corresponding how-to sections to the project documentation, with coverage that answers those questions clearly.

Written by the indexing model from the issue text.