Wonderer to build GrapheneOS the instructions can be found here:
Follow the guidelines for your bootanimation here:
You'll be responsible for updating your own device going forward too.
In all honesty I can tell you now that going to all this effort for a Watchdogs bootanimation is not worth it and would advise against it especially with only a beginners level knowledge. This is not a simple process.
Xiaomi and OnePlus might throw this feature in but they go months without updating their OSes with the latest security patches. This sort of adhoc customisation is antithetical to the project and you must understand that GrapheneOS is not a hobbyist cherry picking style project being run from the well known 3 letter forum.
By all means have a device you can tinker with such things and learn with but your day to day device if using a fully production grade OS like GrapheneOS should be left as is as far as the System is concerned and you can benefit from the prompt and regular updates/upgrades/security patches.