Reorganize documentation and add samples.

[dblock]

Description

• Reduces user guide to a basic starter with links to all advanced topics.
• Standardizes titles and TOCs.
• Adds minimal information about plugins.
• Links all leaf/orphaned guides/documents.
• Adds a working hello world sample that can be run with the current version of code (using poetry).
• Cleanup CHANGELOG

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[codecov]

Codecov Report

Merging #447 (8631c72) into main (5866880) will not change coverage.
The diff coverage is n/a.

❗ Current head 8631c72 differs from pull request most recent head ad7f354. Consider uploading reports for the commit ad7f354 to get more accurate results

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##             main     #447   +/-   ##
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  Coverage   71.49%   71.49%           
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  Files          81       81           
  Lines        7668     7668           
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  Hits         5482     5482           
  Misses       2186     2186