Reorganize directories (#3462)

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

docs/mkdocs/docs/api/ordered_json.md

@@ -0,0 +1,15 @@
+# <small>nlohmann::</small>ordered_json
+
+```cpp
+using ordered_json = basic_json<ordered_map>;
+```
+
+This type preserves the insertion order of object keys.
+
+## See also
+
+- [ordered_map](ordered_map.md)
+
+## Version history
+
+Since version 3.9.0.