FindOpenSceneGraph: Update documentation

- Extended examples section and changed example to use a locally created
  imported target showing how to add usage requirements to project.
- Mention of CMake 2.x removed as these version notes are now being
  cleaned in the CMake 4 documentation (CMake docs are at this point
  considering the 2.8.12.2 as the initial state upon which new and
  changed items are documented through versions).
- Extended components section.
- Added See Also section.
- Removed internal comment about naming conventions. Find modules
  distributed with CMake ideally should follow standard find modules
  naming conventions for variables and to have it synced with
  other find modules shipped with CMake.
- Used the OpenSceneGraph_FOUND variable (CMake 3.3). The uppercased
  OPENSCENEGRAPH_FOUND is also set to the same value.
- Added note in introduction how to include headers.