docs: update expired Discord invite link in contributing.md

[kaihere14]

Fixes #636

Description

• Replaced the expired Discord invite link in contributing.md (https://discord.gg/F9XemadR) with the new active invite: https://discord.gg/sUcS8CWvpS.
• Searched the whole repository for the old invite code. This was the only place it appeared.
• Docs-only change. No code or UI changes.

Resolved or fixed issue

Fixes #636

Screenshots (if applicable)

N/A (documentation change only)

Affirmation

• My code follows the CONTRIBUTING.md guidelines

[kaihere14]

Hey @calvadev this one's a tiny docs-only change: just swapping the expired Discord invite in contributing.md for a working one (closes #636). All 4 checks are green and there's no code touched, so should be a safe merge whenever you get a sec. Thanks!