Fix userguide example in quickstart

[hkershaw-brown]

• Quickstart uses example from pydartdiags package data.
• Added detail on composite observations in user guide to show default composite vs. providing your own yaml.

fixes #124

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.12%. Comparing base (2ecb485) to head (bcf03db).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #129   +/-   ##
=======================================
  Coverage   64.12%   64.12%           
=======================================
  Files           5        5           
  Lines        1115     1115           
=======================================
  Hits          715      715           
  Misses        400      400           

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[mjs2369]

I think the desription of multi-component obs and how to specify them with the yaml file (if not wanting the default) is much clearer and sufficiently fixes the comment made on issue #124

I pushed a commit which I think works better for introducing the get_example_data function in the quickstart. But feel free to remove it if you disagree.

[hkershaw-brown]

nice addition thanks Marlee!