Make cookbook materials fetchables

Follow-up on #84: There should be a single source of truth for cookbook input files (SDFs, PDBs, etc.) instead of having a copy in, e.g., the OpenFE test suite, with a duplicate stored here.

We could then make the first line of each notebook be ! openfe fetch cookbook-files — this could be either a fetchable per-notebook or a single fetchable to download all cookbook notebooks.

This would have the advantage that even a download of a single notebook would be self-contained, instead of requiring users to download a tarball/clone of the repo.

Steps to make this happen:

1. Create the fetchables in the OpenFE CLI. See fetchables.py for examples
2. Cut a release of OpenFE, update ExampleNotebooks to use that OpenFE version.
3. Update the cookbook notebooks here to download the required files.

先从 openfecli/fetchables.py 开始，查看现有的可获取资源模式，然后检查 cookbook notebooks 以及 issue 中提到的重复输入文件。在修改 notebook 设置行之前，协调 OpenFE 的发布和 ExampleNotebooks 中的版本更新。所需的 cookbook 输入能够通过 OpenFE 下载，并且单个 notebook 无需克隆仓库或 tarball 即可运行，即表示完成。