Refactor and improve docs

[MJGaughran]

Simplify code and flesh out docstrings and comments.

[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 43.50%. Comparing base (01ab3ba) to head (1f14f9f).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/deploy_tools/app_builder.py	0.00%	2 Missing ⚠️
src/deploy_tools/layout.py	50.00%	1 Missing ⚠️
src/deploy_tools/sync.py	50.00%	1 Missing ⚠️
src/deploy_tools/validate.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
- Coverage   43.95%   43.50%   -0.46%     
==========================================
  Files          24       23       -1     
  Lines         794      777      -17     
==========================================
- Hits          349      338      -11     
+ Misses        445      439       -6     

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