📝 add more API documentation

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

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+# basic_json::contains
+
+```cpp
+template<typename KeyT>
+bool contains(KeyT && key) const;
+```
+
+Check whether an element exists in a JSON object with key equivalent to `key`. If the element is not found or the JSON
+value is not an object, `#!cpp false` is returned.
+
+## Template parameters
+
+`KeyT`
+:   A type for an object key other than `basic_json::json_pointer`.
+
+## Parameters
+
+`key` (in)
+:   key value to check its existence.
+    
+## Return value
+
+`#!cpp true` if an element with specified `key` exists. If no such element with such key is found or the JSON value is
+not an object, `#!cpp false` is returned.
+
+## Exception safety
+
+Strong exception safety: if an exception occurs, the original value stays intact.
+
+## Complexity
+
+Logarithmic in the size of the JSON object.
+
+## Notes
+
+This method always returns `#!cpp false` when executed on a JSON type that is not an object.
+
+## Example
+
+??? example
+
+    The example shows how `contains()` is used.
+    
+    ```cpp
+    --8<-- "examples/contains.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/contains.output"
+    ```
+
+## Version history
+
+- Added in version 3.6.0.