Update Global Variables page

[ivan-mogilko]

Global Variables page has no screenshots, and dont have thorough explanation of the process:
https://adventuregamestudio.github.io/ags-manual/GlobalVariables.html

Suggestions:

1. Add screenshots. A screenshot of the variable list, and new variable dialog, with explanations of what's what.
2. Explain how to create arrays (supported since 3.6.2 iirc).
3. Remove a section called "What about GlobalInts and Graphical Variables?", these are deprecated for over 15 years.
4. Mention that this panel is for user convenience, and you may declare global variables by hand too. Link an article that explains import/export variables.