[Documentation]: add documentation example for DecompositionSeries

[stephprince]

What would you like changed or added to the documentation and why?

As discussed in the NWB meeting today, DecompositionSeries is not well documented.

DecompositionSeries is meant to store spectral analysis of a time series (e.g., different frequency bands of an LFP signal).

I propose we add a short example to the LFP section of the Extracellular Electrophysiology Data tutorial demonstrating how to use this data type.

Do you have any interest in helping write or edit the documentation?

Yes.

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