docs: add the system widgets design

Four cards: CPU/RAM with every temperature, GPU, filesystems, and the
user cache. Three were asked for; cache became its own card because it
is a different refresh cycle and sharing the disks card would have made
the densest card in the dashboard.

Two findings that shaped it, both from probing the hardware rather than
assuming:

- The Arc B580 exposes no utilisation figure. The xe driver has no
  gpu_busy_percent, intel_gpu_top refuses the device outright, and
  gputop prints per-process rows with ANSI escapes, which is not an
  interface to build on. The card shows temperatures and fan speed and
  no load bar, rather than inventing a number.
- /data is on a spinning disk with no hwmon chip, so there is no sda
  temperature to show. Reading SMART would need smartctl as root and
  would spin up a sleeping disk each poll.

Also: the four NFS mounts are two filesystems behind two servers, so
showing all four printed every number twice. One mount per server.

Both new samplers follow the weather fetch's cache-and-read pattern,
which exists precisely so an unreachable NFS server costs a stale cache
instead of freezing the draw hook.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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