docs: enable nightly-only docsrs (#76)

See rust-lang/rust#43781. This enables infoboxes like
"Available on crate feature XXX only." in the documentation generated
using `cargo doc` on nightly, or by building on docs.rs.

This can be tested with:

```shell
RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --no-deps --all-features
```

flareon/Cargo.toml

@@ -52,6 +52,10 @@ fake.workspace = true
 futures.workspace = true
 mockall.workspace = true
 
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
 [package.metadata.cargo-machete]
 ignored = [
     # askama doesn't seem to be working with minimal versions of its dependencies at the moment,

flareon/src/lib.rs

@@ -41,6 +41,7 @@
     unused_import_braces,
     unused_qualifications
 )]
+#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 
 extern crate self as flareon;