docs: add quickstart guide for TanStack Start

[crutchcorn]

This is an early quick-start guide for TanStack Start. I haven't added it to the routes configuration guide yet, as I'm not sure how public we want this guide to be (not against, just wanting to follow @tannerlinsley's guidance).

This PR is related to and followed up by: https://github.com/TanStack/router/pull/1858

[crutchcorn]

@SeanCassiere thoughts on this guide as a whole? Like, high-level feedback?

[SeanCassiere]

@SeanCassiere thoughts on this guide as a whole? Like, high-level feedback?

As a whole, I'm super down with this doc 👍🏼.

I like that it follows that kind of step-by-step instruction style setup guide. It's what we are already doing 😀.

Maybe this page, in particular, would be better-named something like "Setup TanStack Start" (or something to that effect)? Purely because I think we should probably have some other pages on why TanStack Start exists and what got us here (like the design decisions doc).

Going forward, where possible, I'd probably advocate for shorter pages. Mostly, because the docs site doesn't have a table of contents, which can make scrolling feel a bit infinite (like our current search params page).

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