Move the trademark policy from the wiki to this repo

The current trademark policy is located in the wiki: https://wiki.archlinux.org/title/DeveloperWiki:TrademarkPolicy

Since the code of conduct and the privacy policy were moved here, it would make sense to move the trademark policy as well. (Original discussion) The front page even links to the trademark policy as a separate document.

One issue that comes to mind is that the trademark policy is not a "service agreement", it won't be shown to users upon registration via Keycloak, which was the original purpose of this repo. But the descriptions of the project (which seem to define its scope) are already inconsistent, the published documents at https://terms.archlinux.org/ call it "Arch Linux Terms" or "Arch Linux legal documentation". The issue can be solved by defining the scope of the repo properly.