Reorganize directories (#3462)

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

docs/mkdocs/docs/integration/index.md

@@ -0,0 +1,18 @@
+# Header only
+
+[`json.hpp`](https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json.hpp) is the single required
+file in `single_include/nlohmann` or [released here](https://github.com/nlohmann/json/releases). You need to add
+
+```cpp
+#include <nlohmann/json.hpp>
+
+// for convenience
+using json = nlohmann::json;
+```
+
+to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., `-std=c++11` for GCC and
+Clang).
+
+You can further use file [`include/nlohmann/json_fwd.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/json_fwd.hpp)
+for forward-declarations. The installation of `json_fwd.hpp` (as part of CMake's install step), can be achieved by
+setting `-DJSON_MultipleHeaders=ON`.