Improve documentation around how to use git lfs

[emilk]

We mandate that png images are put on git lfs, and have a CI job to check it. However, we lack clear instructions of how to:

• Add a file to git lfs
• Move a file from "normal" git to lfs
• Move a file from lfs to "normal" git

We should add such instructions to CONTRIBUTING.md and link to it from .github/workflows/png_only_on_lfs.yml (make error messages actionable)

[emilk]

There also seems to be a problem adding commits to user PRs if any of the commits touches git lfs. This is very annoying, and I wonder if there is some way around it.