Reorganize directories (#3462)

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

docs/mkdocs/docs/api/adl_serializer/to_json.md

@@ -0,0 +1,26 @@
+# <small>nlohmann::adl_serializer::</small>to_json
+
+```cpp
+template<typename BasicJsonType, typename TargetType = ValueType>
+static auto to_json(BasicJsonType& j, TargetType && val) noexcept(
+    noexcept(::nlohmann::to_json(j, std::forward<TargetType>(val))))
+-> decltype(::nlohmann::to_json(j, std::forward<TargetType>(val)), void())
+```
+
+This function is usually called by the constructors of the [basic_json](../basic_json) class.
+
+## Parameters
+
+`j` (out)
+:   JSON value to write to
+
+`val` (in)
+:   value to read from
+
+!!! note
+
+    This documentation page is a stub.
+
+## Version history
+
+- Added in version 2.1.0.