Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/back.md

@@ -0,0 +1,65 @@
+# <small>nlohmann::basic_json::</small>back
+
+```cpp
+reference back();
+
+const_reference back() const;
+```
+
+Returns a reference to the last element in the container. For a JSON container `c`, the expression `c.back()` is
+equivalent to
+
+```cpp
+auto tmp = c.end();
+--tmp;
+return *tmp;
+```
+    
+## Return value
+
+In case of a structured type (array or object), a reference to the last element is returned. In case of number, string,
+boolean, or binary values, a reference to the value is returned.
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
+
+## Exceptions
+
+If the JSON value is `#!json null`, exception
+[`invalid_iterator.214`](../../home/exceptions.md#jsonexceptioninvalid_iterator214) is thrown.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+!!! info "Precondition"
+
+    The array or object must not be empty. Calling `back` on an empty array or object yields undefined behavior.
+
+## Examples
+
+??? example
+
+    The following code shows an example for `back()`.
+     
+    ```cpp
+    --8<-- "examples/back.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/back.output"
+    ```
+
+## See also
+
+- [front](front.md) to access the first element
+
+## Version history
+
+- Added in version 1.0.0.
+- Adjusted code to return reference to binary values in version 3.8.0.