doc: update InitFDTD with missing multigrid option

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
new Tutorial: MRI lowpass birdcage coil

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>
new tutorial: a bent patch antenna in cylindrical coordinates

Signed-off-by: Thorsten Liebig <Thorsten.Liebig@gmx.de>