doc: instant navs runtime story (#93204)
- changes getting started -> caching
- new guide for instant navs
- new guide for runtime-prefetching (most pending stuff is here)
- x-refs between docs
- App Shell mentions in other docs (ISR w/ CC)
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Co-authored-by: Aurora Scharff <66901228+aurorascharff@users.noreply.github.com>
Co-authored-by: Aurora Scharff <aurora.sofie@gmail.com>
