Clarify license for DOI logo
The license of the DOI logo, as added in issue #15  , remains unclear.
Are we allowed to use the DOI logo in our works?

- The DOI website provides a [pdf](https://www.doi.org/doi_handbook/pub_agreements/130718Trademark_Policy.pdf) that states `"The International DOI Foundation (“IDF”) owns the trademarks (...) and "[DOI logo]" (the “IDF Trademarks”)."` , and further `"Only the IDF and its authorized licensees may use the IDF Trademarks."`  => No usage rights given.
- The website https://www.doi.org/logo.html provides a gif and states "The logo is provided as a convenience for companies, organizations and the press." => No clear license and usage rights statement.
- The original issue #15 links to a svg from wikipedia. The [wikipedia page](https://commons.wikimedia.org/wiki/File:DOI_logo.svg) states that this image `"does not meet the threshold of originality needed for copyright protection, and is therefore in the public domain"`. However, the logo may be `"protected as a trademark in some jurisdictions. If you want to use it, you have to ensure that you have the legal right to do so"` => public domain is US specific and the concept may not translate to other states. In addition, it is just an assumption. Even ignoring these aspects, the trademark situation remains unclear. Wikipedia may be allowed to use it, but individual users may not.
- Issue #80 contains a similar question for the Elsevier logo, which was not included due to the trademark and unclear usage rights (although the Elsevier logo is more complex).

Is there any further information whether we can use the DOI logo in our works?
Currently, I am inclined to answer this question negatively. 