📝 add more API documentation

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

@@ -0,0 +1,64 @@
+# basic_json::get_ref
+
+```cpp
+template<typename ReferenceType>
+ReferenceType get_ref();
+
+template<typename ReferenceType>
+const ReferenceType get_ref() const;
+```
+
+Implicit reference access to the internally stored JSON value. No copies are made.
+
+## Template arguments
+
+`ReferenceType`
+:   reference type; must be a reference to [`array_t`](array_t.md), [`object_t`](object_t.md),
+    [`string_t`](string_t.md), [`boolean_t`](boolean_t.md), [`number_integer_t`](number_integer_t.md), or
+    [`number_unsigned`](number_unsigned.md), [`number_float_t`](number_float_t.md), or [`binary_t`](binary_t.md).
+    Enforced by static assertion.
+
+## Return value
+
+reference to the internally stored JSON value if the requested reference type fits to the JSON value; throws
+[`type_error.303`](../../home/exceptions.md#jsonexceptiontype_error303) otherwise
+
+## Exception safety
+
+Strong exception safety: if an exception occurs, the original value stays intact.
+
+## Exceptions
+
+Throws [`type_error.303`](../../home/exceptions.md#jsonexceptiontype_error303) if the requested reference type does not
+match the stored JSON value type; example: `"incompatible ReferenceType for get_ref, actual type is binary"`.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+!!! warning
+
+    Writing data to the referee of the result yields an undefined state.
+
+## Example
+
+??? example
+
+    The example shows several calls to `get_ref()`.
+    
+    ```cpp
+    --8<-- "examples/get_ref.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/get_ref.output"
+    ```
+
+## Version history
+
+- Added in version 1.1.0.
+- Extended to binary types in version 3.8.0.