Provide a list of notifications

**Your Godot version:**
N/A

**Issue description:**
The page on notifications doesn't actually tell the reader what the available notifications are, making it rather useless.

I thought a list might be on one of the two constants pages, since constants do exist for them, but nope, not there either.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/best_practices/godot_notifications.html
https://docs.godotengine.org/en/stable/classes/class_@gdscript.html#constants
https://docs.godotengine.org/en/stable/classes/class_%40globalscope.html

Start with the Godot notifications page at tutorials/best_practices/godot_notifications.html, then compare the linked GDScript and GlobalScope constants pages to identify the available notifications. Update the documentation so readers can find a complete list, and verify that the listed notifications match the relevant constants.

Written by the indexing model from the issue text.