Docs: Add required/optional info to folder structure

[josmperez]

Add optional/required info to folder structure section of doc.

Fixes #771, subtask 2

In review, consider whether we are providing enough information to help plugin developers ascertain whether or not they can avoid downloading any extraneous files. Are there files not listed here that could be removed? If so, please add that info into this PR.

[github-actions]

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