### What needs to get done Write a how-to for registering stage-packgaes filters. ### Why it needs to get done The current documentation is only in the auto-generated package reference, and the docstring for that is confusing, as it implies that it is deprecated. https://canonical-craft-parts.readthedocs-hosted.com/en/latest/reference/gen/craft_parts.callbacks.html#craft_parts.callbacks.register_stage_packages_filter