Explain what meta is and you have to supply meta

[nevencaplar]

In documentation, explain what meta is and you have to supply meta. I believe that this information at minimum needs to be a subsection in the 2.3 Applying a function (e.g. map_rows/reduce)
2.4 Working with TimeSeries

@wilsonbb point out that there is meta section for reduce in the nested-dask quickstart, but a) that should be updated with a better example and b) be moved to LSDB docs.

[gitosaurus]

Completed the nested-dask part here lincc-frameworks/nested-dask#95 .