Document str_to_var and ConfigFile security risks

**Your Godot version:**
4.7.1
**Issue description:**
Document str_to_var and ConfigFile security risks, as reported here https://github.com/godotengine/godot/issues/80562.

That way developers know to be careful when using those / avoid using those.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_%40globalscope.html#class-globalscope-method-str-to-var
https://docs.godotengine.org/en/stable/classes/class_configfile.html

Start by reading the linked Godot issue #80562, then inspect the str_to_var section of class_@globalscope.html and class_configfile.html. Update both documentation pages to explain the reported security risks and how developers should use these APIs safely. Done means the warnings are clear, accurate, and visible on both referenced pages.

Written by the indexing model from the issue text.