Consider removing usage of openfe-benchmarks for docs

[IAlibay]

openfe-benchmarks is used to pull ligands through in the example notebooks. However the openfe-benchmarks API is currently deprecated. It would probably be easier to just pull down the relevant ligands as local data?

[mikemhenry]

kartograf/examples/environment.yaml

Line 8 in 21f7483

- git+https://github.com/OpenFreeEnergy/openfe-benchmarks.git

kartograf/.github/workflows/ci.yaml

Line 72 in 21f7483

pip install --no-deps git+https://github.com/OpenFreeEnergy/openfe-benchmarks.git

Agreed, we should at least use @ with some commit or tag!

[mikemhenry]

fixed in #120