Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/begin.md

@@ -0,0 +1,42 @@
+# <small>nlohmann::basic_json::</small>begin
+
+```cpp
+iterator begin() noexcept;
+const_iterator begin() const noexcept;
+```
+
+Returns an iterator to the first element.
+
+![Illustration from cppreference.com](../../images/range-begin-end.svg)
+
+## Return value
+
+iterator to the first element
+
+## Exception safety
+
+No-throw guarantee: this member function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Examples
+
+??? example
+
+    The following code shows an example for `begin()`.
+    
+    ```cpp
+    --8<-- "examples/begin.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/begin.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.