native: documentation improvements

Quite a lot of improvements in the native_posix board
documentation:

* Added TOC
* Reordered
* Given much more emphasis to its limitation
* Added subsection explaining how to overcome some of these
  limitations
* Added rationale section
* Added section comparing this port with QEMU and ISSs
* Corrected build example (this port cannot be compiled on
  Windows, regression from commit
  e62ee6ab )
* Added note about it not working on W10 WSL (#5762)
* Several other minor fixes

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>