Reorganize directories (#3462)

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

docs/mkdocs/docs/api/json.md

@@ -0,0 +1,12 @@
+# <small>nlohmann::</small>json
+
+```cpp
+using json = basic_json<>;
+```
+
+This type is the default specialization of the [basic_json](basic_json/index.md) class which uses the standard template
+types.
+
+## Version history
+
+Since version 1.0.0.