Fix discord link

[webbertakken]

Changes

• Add a redirect from /discord to the discord invite. /discord is used as a shorthand of that very invite and was washed away when we moved from Vercel to Firebase.

See Firebase Hosting priority order to understand how the redirect resolves with regards to the rewrites.

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [x] Readme (updated or not needed)
• [ ] Tests (added, updated or not needed)

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[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 2ddd6c3):

https://game-ci-5559f--pr374-fix-discord-link-844fusw1.web.app

_{(expires Wed, 15 Mar 2023 17:31:30 GMT)}

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