attempt to unbundle `SMul` from `Algebra`

Stream: mathlib4

[Matthew Ballard]

I stubbornly spent too much of my time trying to get a working copy with docs#SMul unbundled from docs#Algebra. I ran out of time but pushed the carnage to branch#mrb/unbundle_smul_from_algebra in case anyone wants to see the consequences. TLDR: this seems to make things worse overall