feat(core/providers): add Gitea/Forgejo login provider

[rubyowo]

☕️ Reasoning

Add Gitea/Forgejo OAuth provider.

🧢 Checklist

• [x] Documentation
• [ ] Tests
• [x] Ready to be merged

🎫 Affected issues

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Name	Status	Preview	Comments	Updated (UTC)
auth-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 7, 2024 2:51pm

1 Ignored Deployment

Name	Status	Preview	Comments	Updated (UTC)
next-auth-docs	⬜️ Ignored (Inspect)	Visit Preview		Feb 7, 2024 2:51pm

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