Add pf.quantreg to function reference

`pf.quantreg` is currently not part of the function reference: https://py-econometrics.github.io/pyfixest/reference/

<img width="778" height="694" alt="Image" src="https://github.com/user-attachments/assets/76611dfd-45a2-4f8d-93ff-33602f338cc6" />