📝 add more API documentation

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

@@ -0,0 +1,58 @@
+# basic_json::diff
+
+```cpp
+static basic_json diff(const basic_json& source,
+                       const basic_json& target);
+```
+
+Creates a [JSON Patch](http://jsonpatch.com) so that value `source` can be changed into the value `target` by calling
+[`patch`](patch.md) function.
+
+For two JSON values `source` and `target`, the following code yields always `#!cpp true`:
+```cpp
+source.patch(diff(source, target)) == target;
+```
+
+## Parameters
+
+`source` (in)
+:   JSON value to compare from
+
+`target` (in)
+:   JSON value to compare against
+
+## Return value
+
+a JSON patch to convert the `source` to `target`
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
+
+## Complexity
+
+Linear in the lengths of `source` and `target`.
+
+## Note
+
+Currently, only `remove`, `add`, and `replace` operations are generated.
+          
+## Example
+
+??? example
+
+    The following code shows how a JSON patch is created as a diff for two JSON values.
+     
+    ```cpp
+    --8<-- "examples/diff.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/diff.output"
+    ```
+
+## Version history
+
+- Added in version 2.0.0.