React Router sign-in-or-up docs

[alexcarpenter]

Important

🔎 Previews:

• https://clerk.com/docs/pr/1821/references/react-router/custom-sign-in-or-up-page
• https://clerk.com/docs/pr/1821/references/react-router/custom-sign-up-page

Explanation:

• React Router sign-in-or-up docs

This PR:

• Replace /docs/references/react-router/custom-signup-signin-pages with /docs/references/expo/web-support/custom-sign-in-or-up-page
• Add /docs/references/react-router/custom-sign-up-page

Checklist

• I have clicked on "Files changed" and performed a thorough self-review
• I have added the "deploy-preview" label and added the preview link(s) to this PR description
• All existing checks pass

[github-actions]

Hey, here’s your docs preview: https://clerk.com/docs/pr/1821