Page dedicated to borrowing, re-entrancy and common patterns

Users are repeatedly confused about `bind/bind_mut`, `base/base_mut`, how to make Rust -> Godot -> Rust calls work, etc. While we do have existing docs about binding and reborrowing, they could be more detailed, motivated by real-world examples.

For inspiration, see:
- https://github.com/godot-rust/gdext/issues/1157#issuecomment-2855306269

Start by reading the existing documentation on binding and reborrowing, then review the linked comment from issue #1157 for real-world motivation. Done means adding a dedicated page that explains bind/bind_mut, base/base_mut, and Rust → Godot → Rust calls with detailed examples.

Written by the indexing model from the issue text.