Added docs on dumping dataclasses to JSON

[mikegrima]

Change Summary

Added docs about dumping dataclasses to JSON. I was frustrated by the lack of them :D

Checklist

• [N/A] Unit tests for the changes exist
• [N/A] Tests pass on CI and coverage remains at 100%
• [Y] Documentation reflects the changes where applicable
• [Y] changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1487 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1487   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3761      3761           
  Branches       748       748           
=========================================
  Hits          3761      3761           

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[samuelcolvin]

thanks a lot.