Documentation: net: dsa: transition to the rst format

This patch also performs some minor adjustments such as numbering for
the receive path sequence, conversion of keywords to inline literals and
adding an index page so it looks better in the output of 'make htmldocs'.

Signed-off-by: Ioana Ciornei <ciorneiioana@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>