DOC: Flight Class docs page still lacking!

[Gui-FernandesBR]

... Still lacking, although it's important

[MateusStano]

With the current state of the documentation I think a more in depth Flight Class docs page would be really important!

[Gui-FernandesBR]

Already started here: https://github.com/RocketPy-Team/RocketPy/tree/doc/documentation-improvements