Relation to nightly Rust

Thanks for building these crates! I am tempted to experiment with them, most likely in PyO3's CI initially - we have a [`pyo3-ffi-check`](https://github.com/PyO3/pyo3/tree/20246a895718baa603f836b187b498cedc6941ca/pyo3-ffi-check) helper which compares our hand-maintained FFI to bindgen (because reasons).

Currently has a ton of hacks where I build docs html and parse that, which is painful plus has shortcomings like case-insensitive filesystems meaning I can't easily check the difference between e.g. `Py_INCREF` and `Py_IncRef` symbols.

I understand that the rustdoc JSON is still not stable. Do these crates automate the process of pinning / selecting the right nightly toolchain to get "stable" rustdoc JSON processing?

Start with the pyo3-ffi-check helper linked in the issue and review how it currently depends on generated documentation HTML. Then inspect the crates' rustdoc JSON and nightly-toolchain entry points to determine whether pinning or selecting a compatible nightly is supported. Done means documenting the current capability and, if absent, defining the required automation and compatibility behavior.

Written by the indexing model from the issue text.