ENH: Categorical.from_union
xref #13361
- [x] support union w `Series/CategoricalIndex` as well as `Categorical` #14199
- [x] add `ignore_order` to ignore the raising on an ordered Categorical (and just have it work) #15219 
- [ ] do we want to put this in the `pd` namespace (or change its name).  Consider `Categorical.from_union(...)`
