docs: add hardware-in-the-loop rig reference

Document the ci and hfp HIL rigs in enough detail to reproduce one: bill of
materials with photos, BIOS/IOMMU and vfio-pci passthrough on the Proxmox
host, the Renesas uPD720201 firmware install, the guest software and
permissions, the one-hub-per-root-port USB topology rule and the per-box
split of probe and DUT hubs, how CI drives the rigs, and the operational
gotchas.

The attached-board table is generated from test/hil/tinyusb.json and
test/hil/hfp.json by tools/gen_doc.py into docs/reference/hil_boards.md,
which the page includes. Sphinx excludes that partial so it is not also
built as an orphan document.

Also exclude docs/superpowers/ from the Sphinx build: it holds internal
plans, specs and handoffs rather than published documentation, and since
nothing references them from a toctree each emitted "document isn't
included in any toctree" -- 26 warnings in total, so build_doc.py -W could
never pass. It now does.

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