📝 add more API documentation

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

@@ -0,0 +1,58 @@
+# basic_json::merge_patch
+
+```cpp
+void merge_patch(const basic_json& apply_patch);
+```
+
+The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of
+modifications to a target resource's content. This function applies a merge patch to the current JSON value.
+
+The function implements the following algorithm from Section 2 of
+[RFC 7396 (JSON Merge Patch)](https://tools.ietf.org/html/rfc7396):
+
+```python
+define MergePatch(Target, Patch):
+  if Patch is an Object:
+    if Target is not an Object:
+      Target = {} // Ignore the contents and set it to an empty Object
+    for each Name/Value pair in Patch:
+      if Value is null:
+        if Name exists in Target:
+          remove the Name/Value pair from Target
+      else:
+        Target[Name] = MergePatch(Target[Name], Value)
+    return Target
+  else:
+    return Patch
+```
+
+Thereby, `Target` is the current object; that is, the patch is applied to the current value.
+
+## Parameters
+
+`apply_patch` (in)
+:   the patch to apply
+
+## Complexity
+
+Linear in the lengths of `apply_patch`.
+
+## Example
+
+??? example
+
+    The following code shows how a JSON Merge Patch is applied to a JSON document.
+     
+    ```cpp
+    --8<-- "examples/merge_patch.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/merge_patch.output"
+    ```
+
+## Version history
+
+- Added in version 3.0.0.