Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/empty.md

@@ -0,0 +1,66 @@
+# <small>nlohmann::basic_json::</small>empty
+
+```cpp
+bool empty() const noexcept;
+```
+
+Checks if a JSON value has no elements (i.e. whether its [`size()`](size.md) is `0`).
+    
+## Return value
+
+The return value depends on the different types and is defined as follows:
+
+| Value type | return value                           |
+|------------|----------------------------------------|
+| null       | `#!cpp true`                           |
+| boolean    | `#!cpp false`                          |
+| string     | `#!cpp false`                          |
+| number     | `#!cpp false`                          |
+| binary     | `#!cpp false`                          |
+| object     | result of function `object_t::empty()` |
+| array      | result of function `array_t::empty()`  |
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant, as long as [`array_t`](array_t.md) and [`object_t`](object_t.md) satisfy the
+[Container](https://en.cppreference.com/w/cpp/named_req/Container) concept; that is, their `empty()` functions have
+constant complexity.
+
+## Possible implementation
+
+```cpp
+bool empty() const noexcept
+{
+    return size() == 0;
+}
+```
+
+## Notes
+
+This function does not return whether a string stored as JSON value is empty -- it returns whether the JSON container
+itself is empty which is `#!cpp false` in the case of a string.
+
+## Examples
+
+??? example
+
+    The following code uses `empty()` to check if a JSON object contains any elements.
+    
+    ```cpp
+    --8<-- "examples/empty.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/empty.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.
+- Extended to return `#!cpp false` for binary types in version 3.8.0.