docs: add link to cookbook

www/docs/guide/system-customization.md

@@ -25,6 +25,7 @@ exclude = [...]
 
 A recipe describes modifications to the system.
 Rugpi Bakery comes with a set of [builtin recipes](https://github.com/silitics/rugpi/tree/main/recipes) which you can use.
+Additional guides and recipes for reoccurring problems are also described in the [Rugpi Cookbook](../cookbook).
 
 Each recipe has its own directory with a `recipe.toml` configuration file.
 Recipes may have a `description`, a `priority`, and `dependencies`.