docs/projects/ad719x_asdz: Update documentation

 * Added GPIO section as it was missing
 * Added details on using the make parameter IO_PMOD_N
 * Removed the diagram from Software considerations as it didn't make
   sense, since the evaluation board which doesn't have PMOD connector, will
   be placed on top of Cora to use the I/O header, and not connected through
   wires
 * Added links to Linux and no-OS files

Signed-off-by: Iulia Moldovan <[email protected]>