I just made a wrapper script that calls the script in the serenity repo (which I cloned into my home directory) and put the script in PATH e.g. in /usr/bin/ladybird. The content my script.

> #!/usr/bin/env bash

> cd ${HOME}/serenity && ./Meta/serenity.sh run lagom ladybird

I guess you could create a .desktop file that invokes the script, or just the "serenity.sh" script directly.