DOC: Better document spectrogram options, e.g. with a how-to + link to API + reference docs

[NickleDave]

Unclear from the docs right now how spectrogram can be configured, see this question in the vocalpy forum:
https://forum.vocalpy.org/t/specifying-the-frequency-range-using-vak/67/2

We do document the section of the config file using the class that represents it but there's zero narrative text around it: https://vak.readthedocs.io/en/latest/reference/config.html#spect-params-section

One could also find this function's docstring in the API doc if they looked hard enough but there's not much there either: https://vak.readthedocs.io/en/latest/api/generated/vak.spect.spectrogram.html#vak.spect.spectrogram

We should instead add to the how-to guide here:
https://vak.readthedocs.io/en/latest/howto/index.html