📝 add more API documentation

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

@@ -0,0 +1,15 @@
+# basic_json::get_allocator
+
+```cpp
+static allocator_type get_allocator();
+```
+
+Returns the allocator associated with the container.
+    
+## Return value
+
+associated allocator
+
+## Version history
+
+- Unknown.