Create a k4FWCore overview page

[tmadlener]

We should have a page that briefly introduces k4FWCore, including

• Purpose of k4FWCore
• Overview of components of k4FWCore
  • Cross-link to How-To section detailing how to write a Functional algorithm
  • Table of which Functional to choose when(?)
  • Include the non-algorithm parts (e.g. random service)