This PR updates the `/rustdoc` slash command to use local docs built
with `cargo doc`.

If the docs for a particular crate/module are available locally, those
will be used. Otherwise, it will fall back to retrieving the docs from
`docs.rs`.

The placeholder output for the slash command will indicate which source
was used for the docs:

<img width="289" alt="Screenshot 2024-06-03 at 4 13 42 PM"
src="https://github.com/zed-industries/zed/assets/1486634/729112e4-80ca-4f08-bdb3-88fc950351c3">

Release Notes:

- N/A