Returns a PDF copy of a piece of evidence attached to a business profile.

Read evidence in this order:

1. At its originalURL.
2. If that no longer works, at its assetURL, when it is set.
3. Otherwise, with this endpoint. This is always the case for evidence
   marked isCopyrighted, which has no assetURL.

Take the evidenceID from the evidences list of the business profile
returned by GET /businesses/{id}.

The response is the PDF itself, not a JSON:API document. A 404 means
that no PDF is available for this evidence.