docs: add general build docs

[dummdidumm]

closes #8500 alternative to #8500

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[benmccann]

If we go with this version, I'd put the new page under Advanced. This is basically just docs for import { building } from '$app/environment', which feels out of place in Core Concepts.

[dummdidumm]

Building your app is a core concept, so it feels weird to me to put it under advanced, especially if thereby adapter appears first in the list.

[Rich-Harris]

closing in favour of https://github.com/sveltejs/kit/pull/8531