feat: add authentication quickstart

[joshzhang5]

Pull Request Checklist

• Did you add new tests and confirm existing tests pass? (yarn test)
• Did you update relevant docs? (docs are found in the site folder, and guidelines for updating/adding docs can be found in the contribution guide)
• Do your commits follow the Conventional Commits standard?
• Does your PR title also follow the Conventional Commits standard?
• If you have a breaking change, is it correctly reflected in your commit message? (e.g. feat!: breaking change)
• Did you run lint (yarn lint:check) and fix any issues? (yarn lint:write)
• Did you follow the contribution guidelines?

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PR-Codex overview

This PR focuses on adding a new Setup page for authentication in the Dashboard, enhancing user guidance on enabling login methods and configuring OAuth settings.

Detailed summary

• Added a new Setup page in docs/pages/authentication/setup.mdx.
• Updated docs/docs.yml to include the new Setup page.
• Revised instructions for enabling login methods and policies in docs/pages/react/getting-started/setup.mdx.
• Included images for social auth and redirect URI setup.

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