Document how to use rust-toolchain.toml to avoid breaking changes in CI

I've had Clippy suddenly start to fail in GitHub Actions for me in multiple ways.

If Clippy was still a crate, I'd just pin it as a dependency and run https://crates.io/crates/cargo-run-bin/1.7.2

However, that's unfortunately not possible and I found it far from obvious what to do instead. My understanding is that pinning it using `rust-toolchain.toml` like this is the "correct" way to avoid breaking changes in CI. This took some time and research (and help from @obi1kenobi) to figure out and I'd like to save others the trouble by documenting it.

Author: lgarron

book/src/installation.md

@@ -15,6 +15,16 @@ If Clippy was not installed for a toolchain, it can be installed with
 $ rustup component add clippy [--toolchain=<name>]
 ```
 
+### Use a specific version of Clippy
+
+Clippy may introduce new warnings and errors between associated Rust versions. This may be desirable if you want to learn about newly detectable issues in your codebase over time, but it can cause [continuous integration](./continuous_integration/index.md) to fail even if you haven't touched your Rust code. If you'd like to keep Clippy's behaviour stable for your project, use [`rust-toolchain.toml`](https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file) to pin your entire Rust toolchain. For example:
+
+```toml
+[toolchain]
+channel = "1.83.0"
+components = ["clippy"]
+```
+
 ## From Source
 
 Take a look at the [Basics] chapter in the Clippy developer guide to find step-by-step