lib.trivial.importTOML: add example

Friend came to me with misunderstanding of how this function works.
After discussion we came to the conclusion having an example would have
prevented this.

lib/trivial.nix

@@ -642,6 +642,35 @@ in {
   /**
     Reads a TOML file.
 
+    # Examples
+    :::{.example}
+    ## `lib.trivial.importTOML` usage example
+
+    example.toml
+    ```toml
+    title = "TOML Example"
+
+    [hello]
+    world = "foo"
+
+    [hello.bar]
+    foobar = true
+    ```
+
+    ```nix
+    importTOML ./example.toml
+    => {
+      title = "TOML Example";
+      hello = {
+        world = "foo";
+        bar = {
+          foobar = true;
+        };
+      };
+    }
+    ```
+
+    :::
 
     # Inputs