📝 add more API documentation

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

@@ -0,0 +1,58 @@
+# basic_json::clear
+
+```cpp
+void clear() noexcept;
+```
+
+Clears the content of a JSON value and resets it to the default value as if [`basic_json(value_t)`](basic_json.md) would
+have been called with the current value type from [`type()`](type.md):
+
+Value type  | initial value
+----------- | -------------
+null        | `null`
+boolean     | `false`
+string      | `""`
+number      | `0`
+binary      | An empty byte vector
+object      | `{}`
+array       | `[]`
+
+Has the same effect as calling
+
+```.cpp
+*this = basic_json(type());
+```
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Linear in the size of the JSON value.
+
+## Notes
+
+All iterators, pointers and references related to this container are invalidated.
+
+## Example
+
+??? example
+
+    The example below shows the effect of `clear()` to different
+    JSON types.
+    
+    ```cpp
+    --8<-- "examples/clear.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/clear.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.
+- Added support for binary types in version 3.8.0.