Pull request type

• ReadMe, Docs and GitHub updates

Checklist

• Tests for the changes have been added (not needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

The example notebooks are not executed when building the documentation.
There's no way to add custom widgets (i.e. "seealso", "important", etc.)

New behavior

Two .ipynb files were converted to .rst files.
Sphinx will now execute the code cells whenever we build the docs.

• It prevents our documentation to stay outdated.
• This increases building time a little, but the result is worth it imo.

Breaking change

• No

Additional information

• Please check the read the docs, if possible.