[DO MERGE] Sign-in-or-up docs

[alexcarpenter]

Important

🔎 Previews:

<SignIn />

• https://clerk.com/docs/pr/1772/components/authentication/sign-in

Next.js:

• https://clerk.com/docs/pr/1772/references/nextjs/custom-sign-in-or-up-page
• https://clerk.com/docs/pr/1772/references/nextjs/custom-sign-up-page

Expo:

• https://clerk.com/docs/pr/1772/references/expo/web-support/custom-sign-in-or-up-page
• https://clerk.com/docs/pr/1772/references/expo/web-support/custom-sign-up-page

TanStack Start:

• https://clerk.com/docs/pr/1772/quickstarts/tanstack-start
• https://clerk.com/docs/pr/1772/references/tanstack-start/custom-sign-in-or-up-page
• https://clerk.com/docs/pr/1772/references/tanstack-start/custom-sign-up-page

Remix:

• https://clerk.com/docs/pr/1772/quickstarts/remix
• https://clerk.com/docs/pr/1772/references/remix/custom-sign-in-or-up-page
• https://clerk.com/docs/pr/1772/references/remix/custom-sign-up-page

React Router:

• https://clerk.com/docs/pr/1772/quickstarts/react-router
• https://clerk.com/docs/pr/1772/references/react-router/custom-sign-in-or-up-page
• https://clerk.com/docs/pr/1772/references/react-router/custom-sign-up-page

Explanation:

Resolves SDKI-747

We are introducing a combined sign-in-or-up flow that will be the default experience using the <SignIn /> component. Users will be able to still opt-in to a custom <SignUp /> page by adding a custom sign-up route along with setting their CLERK_SIGN_UP_URL.

This PR:

• Introduce documentation on combined sign-in-or-up flows using <SignIn /> component
• Introduce redirect https://github.com/clerk/clerk/pull/853

  // redirects/dynamic/docs.jsonc
  {
    "source": "/docs/references/:path/custom-signup-signin-pages",
    "destination": "/docs/references/:path/custom-sign-in-or-up-page",
    "permanent": true,
  },
  // redirects/static/docs.json
  {
    "source": "/docs/references/expo/web-support/custom-signup-signin-pages",
    "destination": "/docs/references/expo/web-support/custom-sign-in-or-up-page",
    "permanent": true
  }

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/1772

[zchsh]

Looks great! 💯 🤩

• 👀 I skimmed all the pages linked in the PR description
• ⛏️ For those PR description links, I had to adjust some signup vs sign-up differences in the URLs, which is pretty nitpicky so def feel free to ignore. I couldn't find any actual broken links in the docs themselves (... do we have a broken link checker?)
• ❓ I found a lil typo-ish bug in the Expo sidebar, I think at some point the useOauth() item got dropped and replaced with a duplicate sign-in-or-up link

[github-actions]

Hey, here’s your docs preview: https://clerk.com/docs/pr/1772