docs: explain docs folder layout

[casperdcl]

Related issues/links

Part of #1579

Checklist

• [x] ~I have performed a self-review of my code~
• [x] ~I have added docstrings in line with the guidance in the developer guide~
• [x] I have updated the relevant documentation
• [x] ~I have implemented unit tests that cover any new or modified functionality~
• [x] ~CHANGELOG.md has been updated with any functionality change~
• [x] Request review from all relevant developers
• [x] Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• [x] The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License
• [x] I confirm that the contribution does not violate any intellectual property rights of third parties

[MargaretDuff]

[MargaretDuff]

Looks good. I might suggest that your added part goes in a new section called e.g. "Documentation folder locations", that goes beneath "Building documentation locally".