Document truthiness of various GDScript data types in one place

**Your Godot version:**
Any.

**Issue description:**
As mentioned by @Calinou on Discord, we should probably provide a table of truthy/falsy characteristics for various data types in GDScript in one centralized place. Currently this information is sparsely mentioned across the class reference and other docs, but an aggregated table can be pretty useful as a quick reference.

Question is where to put it though. GDScript basics, somewhere else, or maybe its own article?

**URL to the documentation page (if already existing):**