Add rustdoc_to_markdown crate (#12445)

This PR adds a new crate for converting rustdoc output to Markdown.

We're leveraging Servo's `html5ever` to parse the Markdown content, and
then walking the DOM nodes to convert it to a Markdown string.

The Markdown output will be continued to be refined, but it's in a place
where it should be reasonable.

Release Notes:

- N/A

Cargo.toml

@@ -76,6 +76,7 @@ members = [
     "crates/rich_text",
     "crates/rope",
     "crates/rpc",
+    "crates/rustdoc_to_markdown",
     "crates/task",
     "crates/tasks_ui",
     "crates/search",
@@ -220,6 +221,7 @@ dev_server_projects = { path = "crates/dev_server_projects" }
 rich_text = { path = "crates/rich_text" }
 rope = { path = "crates/rope" }
 rpc = { path = "crates/rpc" }
+rustdoc_to_markdown = { path = "crates/rustdoc_to_markdown" }
 task = { path = "crates/task" }
 tasks_ui = { path = "crates/tasks_ui" }
 search = { path = "crates/search" }
@@ -288,6 +290,7 @@ heed = { version = "0.20.1", features = [
     "read-txn-no-tls",
 ] }
 hex = "0.4.3"
+html5ever = "0.27.0"
 ignore = "0.4.22"
 indoc = "1"
 # We explicitly disable http2 support in isahc.
@@ -300,6 +303,7 @@ lazy_static = "1.4.0"
 libc = "0.2"
 linkify = "0.10.0"
 log = { version = "0.4.16", features = ["kv_unstable_serde"] }
+markup5ever_rcdom = "0.3.0"
 nanoid = "0.4"
 nix = "0.28"
 once_cell = "1.19.0"