Pending attribution rows

A pull request that adds an asset adds one new file here instead of a row in ATTRIBUTION.md, so two pull requests never edit the same table.

Name it after the change, lowercase with dashes: render-studio-example.md.
Contents: the rows only, in the format of the files table in ATTRIBUTION.md (| File | Author | Source | Licence |); a header line is allowed and ignored.
A file that already has a row in ATTRIBUTION.md or another pending file fails cargo xtask lists. cargo xtask assets counts pending rows as attributed.
After a merge, the Generated files workflow runs cargo xtask fold-pending on main: it appends the rows to ATTRIBUTION.md and deletes the files. Never run it in a pull request.