Move rustdoc-related code to rustdoc crate (#12945)

This PR moves the rustdoc-related code out of `html_to_markdown` and
into the `rustdoc` crate.

Release Notes:

- N/A

crates/assistant/src/slash_command/rustdoc_command.rs

@@ -7,10 +7,10 @@ use assistant_slash_command::{SlashCommand, SlashCommandOutput, SlashCommandOutp
 use fs::Fs;
 use futures::AsyncReadExt;
 use gpui::{AppContext, Model, Task, WeakView};
-use html_to_markdown::convert_rustdoc_to_markdown;
 use http::{AsyncBody, HttpClient, HttpClientWithUrl};
 use language::LspAdapterDelegate;
 use project::{Project, ProjectPath};
+use rustdoc::convert_rustdoc_to_markdown;
 use ui::{prelude::*, ButtonLike, ElevationIndex};
 use workspace::Workspace;