Support for cargo-external-doc

[mathstuf]

The tool is here. It would be nice to have the extra docs supplied with a project built on docs.rs as well. Not sure of the best way to indicate that extra documentation is available though. A Cargo.toml flag?

[JelteF]

This would really be super great. Right now my package is missing most of its documentation. See the links in this section: https://docs.rs/derive_more/0.5.0/derive_more/#generated-code

[9prady9]

Similar is the case with arrayfire-rust documentation. It doesn't generate the documentation from external markdown files and thus tutorial links in docs.rs are broken.

[jyn514]

I think this is out of scope for docs.rs. The supported way to do this is either doc(include = "x.md") (rust-lang/rust#44732), which works today, or doc = include_str!(...) (soon to be merged in rust-lang/rust#78837), which will probably end up replacing it.

docs.rs doesn't run doc-tests so that bit of the tool isn't relevant, and documenting examples is tracked in a separate issue: #238.

[mathstuf]

I haven't investigated/ported my projects yet, but doc(include) is probably sufficient for me at least.