QM section too big

[certik]

@skirpichev reported: QM section is big, ~2 pages, while it shows almost
nothing about symbolic computing... On another hand,
there is no section about sets module, for example.

[asmeurer]

There is stuff on sets, but it got moved to the supplement.

I frankly agree with you here, but I was vetoed by others who felt that it was important. See #109 and linked discussions.

[certik]

I was promoting it, as being important. But we can definitely shorten it, it doesn't have to be that long.