docs: content overriding with test files

[ineshbose]

Reverts/Resolves #662

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[what-the-diff]

PR Summary

• Remove app.config from default content paths
  The default content paths no longer include ${srcDir}/app.config.{js,ts} files.
• Clarify usage of '!' in layerPaths() function
  A comment has been added to explain the purpose of the '!' symbol at the start of some patterns in the layerPaths() function.
• Documentation update for config option
  The documentation has been updated to include instructions for adding JavaScript and Vue files for a directory and to exclude test files if they are stored together with source files.

[atinux]

Could you keep the app.config file @ineshbose ?