DOC: fix positions and coordinate system documentation page

[giovaniceotto]

Pull request type

• ReadMe, Docs and GitHub updates

Checklist

• Docs have been reviewed and added / updated

Current behavior

The Positions and Coordinate Systems documentation page is an amazing resource that explains what is perhaps the most misunderstood part of RocketPy! However, even the greatest of resources can still be improved a bit.

Here are a couple of mistakes I found while reviewing it when working on another PR:

• The section Example with Origin at the Center of Mass of the Positions and Coordinate Systems documentation page was actually just a copy of the section above.
• The image contained in the section also had the + and - signs reversed.
• The image nozzletcomb.png was not used and likely the result of a typo.
• Some minor typos in the text.

New behavior

• All issues listed above were fixed, in different commits.

Breaking change

• No

Additional information

The reviewer may chose to review commit by commit in order to better understand the changes made.

RTD preview link: https://docs.rocketpy.org/en/doc-fix-positions-documentation/

[Gui-FernandesBR]

#445

[Gui-FernandesBR]

Well done