📝 add more API documentation

doc/mkdocs/docs/api/adl_serializer.md

@@ -0,0 +1,31 @@
+# adl_serializer
+
+```cpp
+template<typename, typename>
+struct adl_serializer;
+```
+
+Serializer that uses ADL ([Argument-Dependent Lookup](https://en.cppreference.com/w/cpp/language/adl)) to choose
+`to_json`/`from_json` functions from the types' namespaces.
+
+It is implemented similar to
+
+```cpp
+template<typename ValueType>
+struct adl_serializer {
+    template<typename BasicJsonType>
+    static void to_json(BasicJsonType& j, const T& value) {
+        // calls the "to_json" method in T's namespace
+    }
+
+    template<typename BasicJsonType>
+    static void from_json(const BasicJsonType& j, T& value) {
+        // same thing, but with the "from_json" method
+    }
+};
+```
+
+## Member functions
+
+- **from_json** - convert a JSON value to any value type
+- **to_json** - convert any value type to a JSON value