📝 add more API documentation

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

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+# basic_json::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.
+
+## Example
+
+??? 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.