📝 add more API documentation

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

@@ -0,0 +1,55 @@
+# basic_json::count
+
+```cpp
+template<typename KeyT>
+size_type count(KeyT&& key) const;
+```
+
+Returns the number of elements with key `key`. If `ObjectType` is the default `std::map` type, the return value will
+always be `0` (`key` was not found) or `1` (`key` was found).
+
+## Template parameters
+
+`KeyT`
+:   A type for an object key.
+
+## Parameters
+
+`key` (in)
+:   key value of the element to count.
+    
+## Return value
+
+Number of elements with key `key`. If the JSON value is not an object, the return value will be `0`.
+
+## 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 `0` when executed on a JSON type that is not an object.
+
+## Example
+
+??? example
+
+    The example shows how `count()` is used.
+    
+    ```cpp
+    --8<-- "examples/count.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/count.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.