Missing documentation on the assumptions of the files and file structure coming from `w3c/aria-practices`

[howard-e]

There are strong assumptions included in the build repository which need to be highlighted to make troubleshooting build issues clearer which are currently missing or only described within the code itself.

Example:
A pattern called 'sample' would be located at content/patterns/sample/sample-pattern.html but there isn't an expectation for there to be content/patterns/sample/sample-support.html, unless it is explicitly added to .tracked-html-assets.json. Same with there being an expectation of a practice called sample having a content/practice/sample/sample-practice.html