batman-adv: Convert batman-adv.txt to reStructuredText

Converting the freeform text to parsable reStructuredText, allows the
integration in the sphinx based documentation system of the kernel. It will
therefore be accessible as hypertext under
https://www.kernel.org/doc/html/latest/



Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>