MeshBench-docs: document the board view once #598 lands

[A13xB0]

The board view (#598) adds a window, a verb and three client methods that the published documentation does not know about.

What needs saying

• The window itself — what it is for, and how it differs from the Hardware tab: the tab draws a board so somebody can recognise it and press it; the board view asks whether it is behaving like its own profile.
• node.boardview and wb.board_view(node) / BoardView(ctx, node) / boardView(node).
• board.reset and device.reset().
• board.screenshot's new to — the node's own screen.png is overwritten every call, and to writes a copy that is not.
• The verdicts, which are the part worth explaining rather than listing: agrees, diverged, silent, not modelled, and why the last two are different facts — one is about the board and one is about us.
• What the wiring table cannot see yet. Per-pin direction, level and edge counts are not instrumented, so every GPIO reads not instrumented. Saying so in the documentation matters as much as saying it in the window.

Also worth a line

The board view works for a board whose parts nobody has recorded — every nRF52 profile is one — because the radio table needs no panel. That is a property somebody reading the docs would otherwise assume was a limitation.

Blocked on #598 merging.

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