Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/items.md

@@ -0,0 +1,100 @@
+# <small>nlohmann::basic_json::</small>items
+
+```cpp
+iteration_proxy<iterator> items() noexcept;
+iteration_proxy<const_iterator> items() const noexcept;
+```
+
+This function allows accessing `iterator::key()` and `iterator::value()` during range-based for loops. In these loops, a
+reference to the JSON values is returned, so there is no access to the underlying iterator.
+
+For loop without `items()` function:
+
+```cpp
+for (auto it = j_object.begin(); it != j_object.end(); ++it)
+{
+    std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
+}
+```
+
+Range-based for loop without `items()` function:
+
+```cpp
+for (auto it : j_object)
+{
+    // "it" is of type json::reference and has no key() member
+    std::cout << "value: " << it << '\n';
+}
+```
+
+Range-based for loop with `items()` function:
+
+```cpp
+for (auto& el : j_object.items())
+{
+    std::cout << "key: " << el.key() << ", value:" << el.value() << '\n';
+}
+```
+
+The `items()` function also allows using
+[structured bindings](https://en.cppreference.com/w/cpp/language/structured_binding) (C++17):
+
+```cpp
+for (auto& [key, val] : j_object.items())
+{
+    std::cout << "key: " << key << ", value:" << val << '\n';
+}
+```
+
+## Return value
+
+iteration proxy object wrapping the current value with an interface to use in range-based for loops
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+When iterating over an array, `key()` will return the index of the element as string (see example). For primitive types
+(e.g., numbers), `key()` returns an empty string.
+
+!!! danger "Lifetime issues"
+
+    Using `items()` on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See
+    <https://github.com/nlohmann/json/issues/2040> for more information.
+
+## Examples
+
+??? example
+
+    The following code shows an example for `items()`.
+    
+    ```cpp
+    --8<-- "examples/items.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/items.output"
+    ```
+
+## Version history
+
+- Added `iterator_wrapper` in version 3.0.0.
+- Added `items` and deprecated `iterator_wrapper` in version 3.1.0.
+- Added structured binding support in version 3.5.0.
+
+!!! warning "Deprecation"
+
+    This function replaces the static function `iterator_wrapper` which was introduced in version 1.0.0, but has been
+    deprecated in version 3.1.0. Function `iterator_wrapper` will be removed in version 4.0.0. Please replace all
+    occurrences of `#!cpp iterator_wrapper(j)` with `#!cpp j.items()`.
+
+    You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated
+    function.