chore: add docs.rs metadata to Cargo.toml and use doc_cfg

We use the nightly
[`doc_cfg`](https://doc.rust-lang.org/unstable-book/language-features/doc-cfg.html)
feature to automatically annotate modules as only available with
specific feature flags enabled in the docs. This requires some
metadata in Cargo.toml that was missing.

Author: m4tx

Cargo.toml

@@ -12,6 +12,10 @@ categories = ["embedded", "encoding", "no-std::no-alloc", "parsing"]
 keywords = ["device-tree", "fdt", "dtb", "dts", "parser"]
 readme = "README.md"
 
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
 [features]
 default = []
 write = ["dep:indexmap", "dep:twox-hash"]