Update prerequisites documentation

A few assorted improvements to the prerequisites documentation:

* Rust nightly/beta requirements are removed
* Wording indicating that `wasm-pack` requires `npm` has been clarified
* The dedicated Rust page was removed as it's largely just "install
  Rust" nowadays
* The heading of the NPM page in the sidebar now includes `(optional)`
  to clarify that it's not required.

docs/src/SUMMARY.md

@@ -1,8 +1,7 @@
 # Summary
 
 - [Prerequisites](./prerequisites/index.md)
-  - [Rust](./prerequisites/rust.md)
-  - [npm](./prerequisites/npm.md)
+  - [npm (optional)](./prerequisites/npm.md)
 - [Project Setup](./project-setup/index.md)
   - [Using a Template](./project-setup/using-a-template.md)
   - [Manual Setup](./project-setup/manual-setup.md)

docs/src/prerequisites/index.md

@@ -1,10 +1,19 @@
 # Prerequisites
 
-To run `wasm-pack` you'll need to have both Rust and npm installed and configured.
+First you'll want to [install the `wasm-pack` CLI][wasm-pack], and `wasm-pack
+-V` should print the version that you just installed.
 
-- [Rust](/wasm-pack/book/prerequisites/rust.html)
-- [npm](/wasm-pack/book/prerequisites/npm.html)
+[wasm-pack]: https://rustwasm.github.io/wasm-pack/installer/
 
-In the future, we intend to rewrite the npm registry client bits so that the need
-for a Node runtime is eliminated. If you're excited about that work- you should
-reach out to the maintainers and get involved!
+Next, since `wasm-pack` is a build tool, you'll want to make sure you have
+[Rust][rust] installed. Make sure `rustc -V` prints out at least 1.30.0.
+
+[rust]: https://www.rust-lang.org/tools/install
+
+Finally, if you're using `wasm-pack` to install to publish to NPM, you'll want
+to [install and configure `npm`][npm]. In the future, we intend to rewrite the
+npm registry client bits so that the need for a Node runtime is eliminated. If
+you're excited about that work- you should reach out to the maintainers and get
+involved!
+
+[npm]: npm.html

docs/src/prerequisites/rust.md

@@ -1,35 +0,0 @@
-# Rust
-
-`wasm-pack` is a Command Line Interface tool written in Rust, and distributed with `cargo`.
-As a result, you'll need Rust and `cargo` to use `wasm-pack`.
-
-### Installing Rust and Cargo
-
-To install Rust, visit this [page](https://www.rust-lang.org/en-US/install.html), which will
-walk you through installing Rust and `cargo` on your machine using a tool called `rustup`.
-
-To confirm you have Rust and `cargo` installed, run:
-
-```
-rustc --version
-cargo --version
-```
-
-### Rust Versions
-
-`wasm-pack` depends on a library called `wasm-bindgen`. `wasm-bindgen` requires that you use
-Rust 1.30.0 or higher. This version is currently only available on the `nightly` or `beta`
-channels.
-
-To get the correct version of Rust, you'll use `rustup` a Rust version manager that comes
-bundled with Rust. Run this command to install the latest Rust on the `beta` channel:
-
-```
-rustup install beta
-```
-
-You can set your project directory to always use this version of Rust by running:
-
-```
-rustup override set beta
-```