Start on rustdoc crawler (#12942)

This PR adds a first pass at a rustdoc crawler.

We'll be using this to get information about a crate from the rustdoc
artifacts for use in the Assistant.

Release Notes:

- N/A

---------

Co-authored-by: Richard <richard@zed.dev>

crates/rustdoc/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "rustdoc"
+version = "0.1.0"
+edition = "2021"
+publish = false
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/rustdoc.rs"
+
+[dependencies]
+anyhow.workspace = true
+async-trait.workspace = true
+collections.workspace = true
+fs.workspace = true
+futures.workspace = true
+html_to_markdown.workspace = true
+http.workspace = true
+
+[dev-dependencies]