DOC: Add Flight class usage documentation entry to CHANGELOG

Author: Gui-FernandesBR

CHANGELOG.md

@@ -32,6 +32,7 @@ Attention: The newest changes should be on top -->
 
 ### Added
 
+- DOC: Add Flight class usage documentation and update index [#841](https://github.com/RocketPy-Team/RocketPy/pull/841)
 -  ENH: Add the Coriolis Force to the Flight class [#799](https://github.com/RocketPy-Team/RocketPy/pull/799)
 
 ### Changed