📝 add more API documentation

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

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