Add prebuilt assets documentation and Webpack front-end assets to setuptools builds/PyPI releases

[bartfeenstra]

This fixes #1464

[github-actions]

Uploaded Betty for commit f404ccc. You can download it, test it, and post your feedback in a comment to this pull request 💕

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• Betty Desktop for Windows 🪟

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[github-actions]

Uploaded Betty for commit ed582cb. You can download it, test it, and post your feedback in a comment to this pull request 💕

• Betty Desktop for macOS 🍎
• Betty Desktop for Windows 🪟

These downloads will stop working when the pull request is closed.

[github-actions]

Uploaded Betty for commit ba1ecdf. You can download it, test it, and post your feedback in a comment to this pull request 💕

• Betty Desktop for macOS 🍎
• Betty Desktop for Windows 🪟

These downloads will stop working when the pull request is closed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.55%. Comparing base (5a76e86) to head (ba1ecdf).
Report is 1 commits behind head on 0.3.x.

Additional details and impacted files

@@           Coverage Diff           @@
##            0.3.x    #1530   +/-   ##
=======================================
  Coverage   95.55%   95.55%           
=======================================
  Files         196      196           
  Lines       18847    18847           
  Branches     3604     3604           
=======================================
  Hits        18010    18010           
  Misses        572      572           
  Partials      265      265           

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