feat(providers): add Nextcloud provider

[Podter]

☕️ Reasoning

This PR adds Nextcloud OAuth provider. Nextcloud is an open source, self-hosted file sync & communication app platform. Or "Self-hosted G Suite Alternative"

🧢 Checklist

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

Only tested with SvelteKit. I haven't test with Next.js yet. I don't know if this counts as a pass test.

🎫 Affected issues

None

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

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Name	Status	Preview	Comments	Updated (UTC)
auth-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 12, 2023 0:51am

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Name	Status	Preview	Comments	Updated (UTC)
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