Put together a one page summary documentation and how-to for the wrapper

[tmadlener]

Covers the basic functionality of the MarlinProcessorWrapper, LcioEvent and the converter tools. Compiled from the existing documentation and examples, but removing everything that is not wrapper specific.

BEGINRELEASENOTES

• Create a one page summary, including brief how-tos, for the wrapper and related topics.
  • Compiled from the existing documentation, focusing entirely on things that are part of the wrapper repository, removing things that should be introduced elsewhere (e.g. basics of the Gaudi framework).

ENDRELEASENOTES

• Remove redundant information. (Moved to k4FWCore in Add a how-to page for using podio in-/output k4FWCore#149)
• Find a place in the Key4hep webpage. -> Add a how-to / instructions section and populate it with existing content key4hep-doc#51

[tmadlener]

Created key4hep/k4FWCore#148 to keep track of the "superfluous" information that is currently in the edmConverters.md file