build: provide mdBook in Nix shell

docs/src/getting-started.md

@@ -30,11 +30,11 @@ Do not add board-local READMEs. Add a page here instead, then link it from
 
 ## Build the documentation website
 
-The manual is an [mdBook](https://rust-lang.github.io/mdBook/) project. Build
-it locally with a compatible `mdbook` binary:
+The manual is an [mdBook](https://rust-lang.github.io/mdBook/) project. The
+repository's Nix development shell provides the checked tool version:
 
 ```sh
-make -C docs build
+nix develop -c make -C docs build
 ```
 
 The generated static site is written to `target/docs/`. The GitHub Pages