Reorganize directories (#3462)

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

docs/mkdocs/docs/api/json_pointer/string_t.md

@@ -0,0 +1,12 @@
+# <small>nlohmann::json_pointer::</small>string_t
+```cpp
+using string_t = RefStringType;
+```
+
+The string type used for the reference tokens making up the JSON pointer.
+
+See [`basic_json::string_t`](../basic_json/string_t.md) for more information.
+
+## Version history
+
+- Added in version 3.11.0.