📝 add more API documentation

doc/mkdocs/docs/api/basic_json/operator=.md

@@ -0,0 +1,46 @@
+# basic_json::operator=
+
+```cpp
+basic_json& operator=(basic_json other) noexcept (
+    std::is_nothrow_move_constructible<value_t>::value &&
+    std::is_nothrow_move_assignable<value_t>::value &&
+    std::is_nothrow_move_constructible<json_value>::value &&
+    std::is_nothrow_move_assignable<json_value>::value
+);
+```
+
+Copy assignment operator. Copies a JSON value via the "copy and swap"
+strategy: It is expressed in terms of the copy constructor, destructor,
+and the `swap()` member function.
+
+## Parameters
+
+`other` (in)
+:   value to copy from
+
+## Complexity
+
+Linear.
+
+## Example
+
+??? example
+
+    The code below shows and example for the copy assignment. It
+    creates a copy of value `a` which is then swapped with `b`. Finally\, the
+    copy of `a` (which is the null value after the swap) is
+    destroyed.
+     
+    ```cpp
+    --8<-- "examples/basic_json__copyassignment.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/basic_json__copyassignment.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.