Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/json_serializer.md

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+# <small>nlohmann::basic_json::</small>json_serializer
+
+```cpp
+template<typename T, typename SFINAE>
+using json_serializer = JSONSerializer<T, SFINAE>;
+```
+
+## Template parameters
+
+`T`
+:   type to convert; will be used in the `to_json`/`from_json` functions
+
+`SFINAE`
+:   type to add compile type checks via SFINAE; usually `#!cpp void`
+
+## Notes
+
+#### Default type
+
+The default values for `json_serializer` is [`adl_serializer`](../adl_serializer).
+
+## Version history
+
+- Since version 2.0.0.