docs(adopting-partial-prefetching): add 'Choosing a strategy' table
Six-row reference table organized around what the user wants (app-wide
default, per-route default, per-link upgrade, etc.) and which knob to
set. Drops the dev-internals framing of the earlier ad-hoc grid (no
pre/post PPF columns, no 'enables PPF for that subtree' phrasing).
Also clarifies that unstable_instant = true implicitly opts the route
into partial prefetching, so the two exports don't need to be combined.