FOI: Legacy spec

[effigies]

I've pulled, I think, all of the Google doc into https://bids-standard.github.io/stats-models/legacy.html

I've already removed some stuff that was duplicated elsewhere, but have generally erred on the side of leaving things in. Please make removing things from the legacy spec and adding to the new spec part of the same PR from here on.

The legacy spec does not need to be treated with undue respect. There are things I've noticed that are inconsistent, so if people want to make changes to it prior to converting to proper spec that's fine.

cc @adelavega