DX-077: Write /concepts/oracles

[IbrahimIjai]

Context

Prices come from Binance as primary with a GMX oracle fallback, and Pyth is a
dependency in the web app. Since the oracle determines liquidation, a reader is
entitled to know exactly what they are trusting and how it can fail.

Scope

• Which sources are used, in what priority, and what triggers a fallback —
  sourced from the actual implementation, not from the README summary.
• How often prices update, and what staleness means for an open position.
• The difference between the displayed mark price and the price used for
  execution and liquidation, if they differ.
• Failure modes: source outage, stale price, large deviation — and what the
  system does in each case.
• A diagram of the price path from source to liquidation check.

Acceptance criteria

• The source list and fallback conditions match the code, with file paths.
• The failure-mode section describes real behaviour, not intended behaviour;
  if something is unhandled today, the page says so.
• The diagram renders in both themes with a caption.
• Reviewed by a maintainer for accuracy.

Out of scope

• Oracle design proposals or roadmap items.

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Spec: docs/dx_1/003_content_map.md §3 · Depends on: DX-076

[Bilalishaq7]

Gooday maintainer,I'd love to take on this task, I can fix this and i'll submit a PR shortly after Assignment