[Documentation]: Update DecompositionSeries example for schema v2.8.0

[stephprince]

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

See #1981 (comment)

#1981 added a new section to the ecephys tutorial using DecompositionSeries. However the file generated by the tutorial now violates best practices by generating a column of all nans. Once the band_mean and band_stdev columns are made optional, this example should be updated.

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

Yes.

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