BIRD Debian repo instructions need updating to reflect best-practice

Re: https://bird.network.cz/?download&tdir=debian/ The instructions presented are not inline with Debian best-practices. As per https://wiki.debian.org/DebianRepository/UseThirdParty: "The key MUST be downloaded over a secure mechanism like HTTPS to a location only writable by root, which SHOULD be /usr/share/keyrings. The key MUST NOT be placed in /etc/apt/trusted.gpg.d or loaded by apt-key add. A sources.list entry SHOULD have the signed-by option set. The signed-by entry MUST point to a file, and not a fingerprint."