Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/size.md

@@ -0,0 +1,57 @@
+# <small>nlohmann::basic_json::</small>size
+
+```cpp
+size_type size() const noexcept;
+```
+
+Returns the number of elements in a JSON value.
+    
+## Return value
+
+The return value depends on the different types and is defined as follows:
+
+| Value type | return value                        |
+|------------|-------------------------------------|
+| null       | `0`                                 |
+| boolean    | `1`                                 |
+| string     | `1`                                 |
+| number     | `1`                                 |
+| binary     | `1`                                 |
+| object     | result of function object_t::size() |
+| array      | result of function array_t::size()  |
+
+## 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 `size()` functions have
+constant complexity.
+
+## Notes
+
+This function does not return the length of a string stored as JSON value -- it returns the number of elements in the
+JSON value which is `1` in the case of a string.
+
+## Examples
+
+??? example
+
+    The following code calls `size()` on the different value types.
+    
+    ```cpp
+    --8<-- "examples/size.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/size.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.
+- Extended to return `1` for binary types in version 3.8.0.