Maybe support more crates in /doc command

[upsuper]

It's unclear how we would do this, though.

[upsuper]

One possible way to support more crates is to treat the first level as the crate name, and download search-index.js from the proposed documentation when queried.

[upsuper]

To support this, we may need to fix #29 first, at least so that we can handle non-ASCII content for description clipping. This would probably make #30 unnecessary.

Also, #32 may serve as a prerequisite to provide some infrastructure for lazy loading data from docs.rs and dynamically update.

We may also want to save generated fst into files so that we don't need to re-generate them every time the bot restarts.

[upsuper]

We may also want to save generated fst into files so that we don't need to re-generate them every time the bot restarts.

This is now #35.