website/docs: improve docs for expressions

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

website/docs/expressions/_objects.md

@@ -0,0 +1,11 @@
+- `ak_logger`: structlog BoundLogger. See ([structlog documentation](https://www.structlog.org/en/stable/api.html#structlog.BoundLogger))
+
+    Example:
+
+    ```python
+    ak_logger.debug("This is a test message")
+    ak_logger.warning("This will be logged with a warning level")
+    ak_logger.info("Passing structured data", request=request)
+    ```
+
+- `requests`: requests Session object. See ([request documentation](https://requests.readthedocs.io/en/master/user/advanced/))