DOC: add additional numba example
#### Location of the documentation
 
https://pandas.pydata.org/pandas-docs/stable/user_guide/enhancingperf.html#using-numba

#### Documentation problem

Provide a simpler copy-paste example.

#### Suggested fix for documentation

@TomAugspurger has a nice example of using numba to speed up a `rolling.apply()` operation here https://github.com/TomAugspurger/acon-2020-pandas/blob/master/Numba%20Acceleration.ipynb

Could this or a variant of this be added to the docs? Perhaps to show that you don't need to do `import numba` but can do `engine='numba'`
