Point at the upstream issue behind the internal-storage behaviour

    The README callout and the #2843 changelog entries described what Bambu
    Studio does without saying it is tracked anywhere, so a reader had no
    way to follow it. Both now link bambulab/BambuStudio#10481.

    The #2843 entry also still blamed H2-series and P2S firmware for
    keeping sliced files internally. That was the reading before measuring
    it: the same printers archive in full when sliced in OrcaSlicer, so the
    destination is the slicer's choice, not the printer's. Corrected here
    rather than left to mislead, since 1.2.6b1 is unreleased.