Reorganize directories (#3462)

* 🚚 move files
* 🚚 rename doc folder to docs
* 🚚 rename test folder to tests

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

@@ -0,0 +1,19 @@
+# <small>nlohmann::basic_json::</small>get_allocator
+
+```cpp
+static allocator_type get_allocator();
+```
+
+Returns the allocator associated with the container.
+    
+## Return value
+
+associated allocator
+
+## Version history
+
+- Unknown.
+
+!!! note
+
+    This documentation page is a stub.