docs: Bus emulators: Misc fixes

* Convert png figures to svg
* Remove app.png and the paragraph which elaborated on it.
  (It was very specific to some particular
  user case, and having it did not improve understanding)
* Correct all references to "native_posix drivers" to
  the be just "emulated drivers"
  (they are not native_sim specific)
* Correct path for a file which was moved

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>