docs: add /guides/.../welcome route redirect -> /quick-start

[iyiolacak]

• I have read the Contribution guide
• This PR references an issue (except for typos, broken links, or other minor problems)

Description (required)

Fix a broken link path that leads new users to a 404.

How I encountered it

• From Google I landed on https://www.solidjs.com/guides/getting-started
• That page says: “We’re working on new docs. You can check out our new beginner tutorial here…”
  - The “here” link points to https://docs.solidjs.com/guides/tutorials/getting-started-with-solid/welcome
• That URL returned 404 on the new docs site.

What this PR does

• Adds a route at src/routes/guides/tutorials/getting-started-with-solid/welcome.tsx
• Issues an HTTP redirect to /quick-start (via standard Web Response with Location)
• Includes a client-side fallback (window.location.replace) for SPA navigations

Testing

• Local dev: pnpm dev
• Visiting /guides/tutorials/getting-started-with-solid/welcome redirects to /quick-start
• Client navigation also redirects via the fallback

Related issues & labels

• Suggested label(s) (optional): documentation, bug, good first issue

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[brenelz]

Couldn't we just fix the link on the old docs instead of setting up a redirect here for the bad link?

[iyiolacak]

This would at least guarantee, if this link was in use somewhere else in the internet or in the SolidJS docs.

I am happy to just change the href if you want to but I wasn't sure if this URL was used on somewhere else both in the docs or external web.

[LadyBluenotes]

I believe this can be put in the legacy redirect page - https://github.com/solidjs/solid-docs/blob/main/src/middleware/legacy-routes-redirect.ts

@brenelz