Iconography: Add section on optical alignment

[wpkelso]

We use optical alignment in a number of icons, notably Videos and Music. This isn't currently explained in the iconography section (we do have a section on optical sizing, however), so someone unfamiliar with the concept could be confused if they try to learn by directly looking in the files.

[danirabbit]

I think in videos we do that trick where you put the points of a triangle in a circle and then center the circle

[wpkelso]

I think in videos we do that trick where you put the points of a triangle in a circle and then center the circle

TIL Triangle center is a mathematical concept; neat. Mentioning that or linking to the wikipedia page or something is probably a good idea