Why forward-facing spokes should be "heads in", except on the drive side of the rear wheel?
When you accelerate, the backward going spokes of your rear wheel are put under additional stress; and when you use a disk brake, the forward going spokes are stressed. The lacing guide you cite tries hard to put all the spokes that get these stress peaks on the outside with their heads facing inwards.

Why is this important?
The flange of your hub typically makes a right angle with the axis of the wheel, but the spokes do not. Instead, they angle away towards the center plane of the wheel. This angling of the spokes relative to the hub's flange puts an additional bending force on the spoke's neck. For inner spokes that have their heads going outwards, this bending force adds to the bending force that is caused by pure spoke tension. For the outer spokes, the two bending forces are in opposite direction, partially cancelling each other out. Thus, inner spokes are naturally under higher danger of breaking at the neck than outer spokes.

Putting the spokes which are stressed by accelerating and braking on the outside (head facing inwards) avoids them getting the structural stress that the inner spokes are subject to.



I learned this the hard way, because I used to lace my wheels without considering this effect. At some point, I realized that it was always the inner backwards facing spokes on my rear wheel (stressed in acceleration) that broken when I accelerated hard. Learning from this, I laced my rear wheel with the backward going spokes on the outside, which significantly reduced the frequency of broken spokes.