Start drafting user guide

[tsalo]

Closes #719 and closes #697.

Changes proposed in this pull request:

• Add a new user guide section.
• Start adding contents of the user guide. The focus is on carving up the examples as needed and couching them in lots of explanatory text, with more connections between examples/sections than in the example gallery.

[codecov]

Codecov Report

Merging #727 (b9e31d0) into main (a4a7d16) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #727   +/-   ##
=======================================
  Coverage   88.55%   88.55%           
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  Files          38       38           
  Lines        4368     4368           
=======================================
  Hits         3868     3868           
  Misses        500      500           

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