Document `.reduce()` returning nested data

[hombit]

It would be nice to document recently implemented .reduce() feature to return nested frames. Ideas for the documentation example case:

1. Split light curve to single band light curves, like {"lc.band", "lc.time", "lc.flux"} to {"lc_r.time", "lc_r.flux", "lc_g.time", "lc_r.flux"}.
2. Produce a periodogram {"periodogram.freq", "periodogram.power"} or a structure function
3. Replace light curve with a new one. For example filter data in a non-trivial way (e.g. sigma-clipping) or interpolate the data

Some of those could go to the LSDB docs instead.

[dougbrn]

@hombit this was closed due to the docstring update in #229 , let me know if that feels sufficient. I'm thinking a more substantial example in LSDB is probably a good next step rather than doing something larger in nested-pandas

[hombit]

Sounds good