Documentation change (#3672)

Co-authored-by: Florian Albrechtskirchinger <falbrechtskirchinger@gmail.com>

docs/mkdocs/docs/api/adl_serializer/to_json.md

@@ -17,9 +17,26 @@ This function is usually called by the constructors of the [basic_json](../basic
 `val` (in)
 :   value to read from
 
-!!! note
+## Examples
 
-    This documentation page is a stub.
+??? example
+
+    The example below shows how a `to_json` function can be implemented for a user-defined type. This function is called
+    by the `adl_serializer` when the constructor `basic_json(ns::person)` is called.
+        
+    ```cpp
+    --8<-- "examples/to_json.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/to_json.output"
+    ```
+
+## See also
+
+- [from_json](from_json.md)
 
 ## Version history