feat(wfe-rustlang): add external tool auto-install and cargo-doc-mdx

External cargo tools (audit, deny, nextest, llvm-cov) auto-install
via cargo install if not found on the system. For llvm-cov, the
llvm-tools-preview rustup component is also installed automatically.

New cargo-doc-mdx step type generates MDX documentation from rustdoc
JSON output. Runs cargo +nightly rustdoc --output-format json, then
transforms the JSON into MDX files with frontmatter, type signatures,
and doc comments grouped by module. Uses the official rustdoc-types
crate for deserialization.

wfe-rustlang/src/rustdoc/mod.rs

@@ -0,0 +1,3 @@
+pub mod transformer;
+
+pub use transformer::transform_to_mdx;