add link to rust driver manual (#818)

Author: terakilobyte

source/rust.txt

@@ -22,7 +22,7 @@ You can add the driver to your application to work with MongoDB in
 Rust. Download it from `crates.io <https://crates.io/crates/mongodb>`__
 or set up a runnable project with examples from our Usage Guide.
 
-- `Usage Guide <https://github.com/mongodb/mongo-rust-driver#example-usage>`__
+- `The Rust Driver Manual <https://mongodb.github.io/mongo-rust-driver/manual/>`__
 
 - `API Reference <https://docs.rs/mongodb/latest>`__
 
@@ -31,6 +31,7 @@ or set up a runnable project with examples from our Usage Guide.
 - `Source Code <https://github.com/mongodb/mongo-rust-driver/>`__
 
 
+
 Installation
 ------------