Document custom error pages

docs/docs/controllers/custom-errors.md

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+# Custom error pages
+
+When a controller returns an HTTP error code higher than 399, like HTTP 404 or HTTP 500 for example, Rwf displays an error page.
+The default error page doesn't provide much information beyond the error code and name, so it's a good idea to use custom ones that would direct your users to helpful resources.
+
+## Custom 404
+
+The most common error your users will run into is requesting a page that doesn't exist. Customizing this error message can be done by providing a catch-all (or wildcard) controller:
+
+```rust
+use rwf::prelude::*;
+
+#[derive(Default)]
+struct NotFound;
+
+#[async_trait]
+impl Controller for NotFound {
+    async fn handle(&self, request: &Request) -> Result<Response, Error> {
+        render!(request, "templates/404.html")
+    }
+}
+```
+
+To configure the Rwf HTTP server to serve your custom `404.html` template, map your `NotFound` controller to the `/*` route, like so:
+
+```rust
+Server::new(vec![
+    // ... your other routes ...
+    NotFound::default().wildcard("/"),
+]);
+```
+
+Wildcard routes have a low rank of -20. When matching requests to controllers, the HTTP server will attempt all other routes, and if none of them match, it will serve the wildcard route.
+
+!!! note
+    If your controllers return HTTP 404 manually, the server will not use your wildcard route and will
+    return the default error page instead. Universal catchers for error codes are on the roadmap.