Explain recommonmark's auto_toc_tree and give an example to make writing documentation easier. Show an example what happens if the document isn't included in any toctree. Change-Id: I4938d8d292ea890caec6d396b4fa04da65e398f4 Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com> Reviewed-on: https://review.coreboot.org/28427 Tested-by: build bot (Jenkins) <no-reply@coreboot.org> Reviewed-by: Tom Hiller <thrilleratplay@gmail.com> Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>