Update Working with CMake GDExtension documentation (#11370)

- Respect the 80 column rule.
- Be more explicit in code blocks.
- Re-arrange for faster grokking.
- Remove half-made tested toolchain section.
- Fix /s/mutially inclusive/mutially exclusive.
- Remove old warning.
- Update examples.

Co-authored-by: A Thousand Ships <96648715+AThousandShips@users.noreply.github.com>
Co-authored-by: Lukas Tenbrink <lukas.tenbrink@gmail.com>

Author: 3 people