Datasets:
Only document sources that are actually committed
The "Sources on main, not yet released" table added in #29 lists parlamint_pl, linking data/parlamint_pl/parlamint_pl.md — a path that does not exist on main and 404s for every reader. make_docs.py reads the working tree, so an uncommitted data/ directory in whoever regenerates the docs becomes a published claim about the main branch.
make_docs.py now asks git which data/ directories are committed and leaves the rest out of the README, naming them on stdout instead. The datasheet is still written before the check, so a contributor can generate it in the same commit that adds the data. The check disables itself when git cannot answer.
Also drops parlamint_pl from the CHANGELOG Unreleased entry, which made the same claim.
Regeneration is otherwise byte-identical: the only README change is the removed row.