📝 add more API documentation

doc/mkdocs/docs/api/basic_json/to_msgpack.md

@@ -0,0 +1,57 @@
+# basic_json::to_msgpack
+
+```cpp
+// (1)
+static std::vector<std::uint8_t> to_msgpack(const basic_json& j);
+
+// (2)
+static void to_msgpack(const basic_json& j, detail::output_adapter<std::uint8_t> o);
+static void to_msgpack(const basic_json& j, detail::output_adapter<char> o);
+```
+
+Serializes a given JSON value `j` to a byte vector using the MessagePack serialization format. MessagePack is a binary
+serialization format which aims to be more compact than JSON itself, yet more efficient to parse.
+
+1. Returns a byte vector containing the MessagePack serialization.
+2. Writes the MessagePack serialization to an output adapter.
+
+## Parameters
+
+`j` (in)
+:   JSON value to serialize
+
+`o` (in)
+:   output adapter to write serialization to
+
+## Return value
+
+1. MessagePack serialization as byte vector
+2. /
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
+
+## Complexity
+
+Linear in the size of the JSON value `j`.
+
+## Example
+
+??? example
+
+    The example shows the serialization of a JSON value to a byte vector in MessagePack format.
+     
+    ```cpp
+    --8<-- "examples/to_msgpack.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/to_msgpack.output"
+    ```
+
+## Version history
+
+- Added in version 2.0.9.