docs(fp8): refresh FP8 Storage page after hook-based loader change (#9241)

PR #9231 routed every nn.Module — including diffusers ModelMixin — through
InvokeAI's `register_forward_pre_hook` / `register_forward_hook` path, but
the FP8 Storage docs still described the old `enable_layerwise_casting`
implementation. Also corrects two unrelated inaccuracies the rewrite
surfaced: pre-Ampere CUDA cards are not a no-op (the FP8 path gates only
on `device.type == "cuda"`, and `float8_e4m3fn` is a pure storage dtype
that works on any CUDA device), and the UI does not grey out the toggle
based on hardware.

Tell users what to include when reporting an FP8 problem so triage isn't
blocked on follow-up questions: repro steps, exact model + variant,
LoRA stack, partner toggles (low-VRAM, cpu_only), GPU + VRAM, OS, and
the relevant log lines.