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>

Cargo.toml

@@ -79,6 +79,7 @@ members = [
     "crates/rich_text",
     "crates/rope",
     "crates/rpc",
+    "crates/rustdoc",
     "crates/task",
     "crates/tasks_ui",
     "crates/search",
@@ -227,6 +228,7 @@ dev_server_projects = { path = "crates/dev_server_projects" }
 rich_text = { path = "crates/rich_text" }
 rope = { path = "crates/rope" }
 rpc = { path = "crates/rpc" }
+rustdoc = { path = "crates/rustdoc" }
 task = { path = "crates/task" }
 tasks_ui = { path = "crates/tasks_ui" }
 search = { path = "crates/search" }