Show that left adjoints preserve specific colimits

[TOTBWF]

In Cat.Functor.Adjoint.Continuous we show that left adjoints preserve colimits generally, but don't show that they preserve specific colimits (coproducts, coequalisers, etc).

This should be pretty mechanical, and any prospective implementor could look at the limit versions of these proofs right here, so this should make a good beginner issue :)

This is blocked on #78.

Checklist

• Initial Objects
• Coproducts
• Coequalisers
• Pushouts