Start work on genericizing /rustdoc (#13745)

This PR begins the process of making the backing infrastructure for the
`/rustdoc` command more generic such that it can be applied to
additional documentation providers.

In this PR we:

- Rename the `rustdoc` crate to `indexed_docs` as a more general-purpose
name
- Start moving rustdoc-specific functionality into
`indexed_docs::providers::rustdoc`
- Add an `IndexedDocsRegistry` to hold multiple `IndexedDocsStore`s (one
per provider)

We haven't yet removed the rustdoc-specific bits in the `DocsIndexer`.
That will follow soon.

Release Notes:

- N/A

crates/indexed_docs/Cargo.toml

@@ -0,0 +1,35 @@
+[package]
+name = "indexed_docs"
+version = "0.1.0"
+edition = "2021"
+publish = false
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/indexed_docs.rs"
+
+[dependencies]
+anyhow.workspace = true
+async-trait.workspace = true
+collections.workspace = true
+derive_more.workspace = true
+fs.workspace = true
+futures.workspace = true
+fuzzy.workspace = true
+gpui.workspace = true
+heed.workspace = true
+html_to_markdown.workspace = true
+http.workspace = true
+indexmap.workspace = true
+parking_lot.workspace = true
+paths.workspace = true
+serde.workspace = true
+strum.workspace = true
+util.workspace = true
+
+[dev-dependencies]
+indoc.workspace = true
+pretty_assertions.workspace = true