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 <[email protected]>
Co-authored-by: Lukas Tenbrink <[email protected]>

Author: 3 people