Add cooldowns tutorial page

[ahmadmsaleem]

Summary

• Adds a new "Cooldowns" tutorial page to the scripting section
• Covers command cooldowns, item cooldowns, variable-based cooldowns, and delay-based warmups
• Includes practical examples, tips, and a comparison summary table
• Adds the page to the scripting index

Sources used as an inspo:

• https://docs.skunity.com/guides/guide/skript-cooldown-for-beginners
• https://docs.skunity.com/guides/tutorial/19354