[docs] add SEO docs

[benmccann]

We talk about SEO on the homepage, so should clarify the benefits SvelteKit offers

I think this is enough that we could consider whether we still want to leave #1142 open. It's not exactly clear to me what else we could provide beyond this documentation in terms of site maps

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[gavinr]

This looks good to me and I'm happy to close/resolve #1142 when it lands. Thank you.

[Rich-Harris]

This is a useful page to have, but we can make it waaaay more concise. I agree with @Conduitry's feedback — this isn't a place to put grandiose marketing claims

[Rich-Harris]

I think we should probably leave #1142 open — we could probably do something to e.g. make prerendered pages available to a sitemap generator. It wouldn't be trivial, and it wouldn't cover all cases, but there's room for Kit to provide value

[Rich-Harris]

This is appendixy content, it should probably go near the bottom (after typescript, perhaps?)