Add binning to example notebook

[ntessore]

Add a section on how to do angular binning to the examples. This is probably best added to the main notebook as most of it would otherwise repeat the same steps.

We can show both the after-the-fact binning

binned_spectra = heracles.binned(spectra, bins=..., weights=...)

and the on-the-fly binning

binned_spectra = heracles.angular_power_spectra(alms, ..., bins=..., weights=...)

for both spectra and mixing matrices.