DOC: Sections in user guide

[NickleDave]

In #99 I moved stuff into a user guide section, but there's, like, not anything in that section. Here are some initial thoughts on what we need.

guides

• spectrograms
• segmentation
• feature extraction
• clustering
  • UMAP
• similarity
  • acoustic
  • sequence
• sharing datasets

how-tos

(this list is def a WIP)

• using acoustic features with scikit-learn
• vignettes with specific algorithms
  • specific segmenting algorithms -- like this example of the ava segmenter: https://github.com/vocalpy/vocalpy/blob/commit-notebooks-in-tests-with-ava-vignette/tests/scripts/vignette-ava-segment.ipynb