📝 add more API documentation

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

@@ -0,0 +1,39 @@
+# basic_json::is_null
+
+```cpp
+constexpr bool is_null() const noexcept;
+```
+
+This function returns `#!cpp true` if and only if the JSON value is `#!json null`.
+    
+## Return value
+
+`#!cpp true` if type is `#!json null`, `#!cpp false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this member function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Example
+
+??? example
+
+    The following code exemplifies `is_null()` for all JSON types.
+    
+    ```cpp
+    --8<-- "examples/is_null.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/is_null.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.