Cheatsheet / translation table

It was brought up on Discord that sometimes, it's not clear how an existing GDScript concept maps to Rust. This is especially true for all the domain-specific syntax, like `$Node`, `export`, etc.

The format could be a simple table with a left column **GDScript** and a right one **Rust**. This would not replace the tutorial parts of the book, as it wouldn't contain explanations (except maybe very brief ones). Deeper explanations of concepts should be elsewhere. If the table grows, it could be split in separate tables depending on category.

Start by reviewing the book's existing tutorial sections and locating where a cheatsheet would fit. Compare the documented GDScript concepts, including domain-specific syntax such as `$Node` and `export`, with their Rust counterparts. Done means a concise two-column GDScript/Rust table exists without replacing the tutorials, with categories added if its scope requires them.

Written by the indexing model from the issue text.