➕ 👷🏻‍♂️ Check for unused files

[ebouchut]

Blocked by #87

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This PR adds the ability to check for the presence of files that are not referenced in the Markdown files:

• locally: you can manually check using this command:

  CHECK_UNUSED_FILES=true mkdocs build -s
  

• CI: On GitHub, before the main branch (dev) is deployed, the CI checks for unused files and fails if any.

Source: https://github.com/wilhelmer/mkdocs-unused-files

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