Add repo.librewolf.net

Background: LibreWolf is a custom Firefox build with stricter privacy and security settings, including accompanying patches. Until now, distro-specific builds for Debian-based distros have been published through a custom repository at https://deb.librewolf.net. Recently, there was an effort though to move to "distroless" builds, only having a single .deb published per version. This will need pretty fundamental changes to the repository structure, so the maintainers (including me) have decided to set up a completely new repository, and migrate existing users to there. During the same time, we also received a suggestion to use extrepo to manage our repository, and thus we want to combine those efforts to simplify things.

So this is adding the new LibreWolf repository (https://repo.librewolf.net) to extrepo. Currently, the title on the HTML index still says "staging" because we only want to start telling users to move to this repository when it is possible to do with extrepo.