DX-074: Write /concepts/liquidation

[IbrahimIjai]

Context

The single most important page in the documentation, and the one where vagueness
does real financial harm. A reader must leave able to compute their own
liquidation price and knowing exactly what they keep and what they lose.

Scope

• Maintenance margin: what it is, what the threshold is, and where it is
  enforced in the contracts.
• How liquidation price is computed, with the formula and a worked example in
  both directions.
• What happens during liquidation: order of operations, fees charged, what
  remains of the collateral.
• Whether partial liquidation exists, and if so its exact conditions. If it does
  not, say so plainly.
• How the interface displays the liquidation price and how oracle behaviour
  (DX-077) can move it.

Acceptance criteria

• Every formula and threshold is verified against contract source — name the
  file, function, and line in the PR description.
• Worked examples are computed and checked against a simulated position.
• The "what you keep" section is unambiguous and unflattering where it needs
  to be.
• Reviewed by a maintainer for accuracy before merge.

Out of scope

• Liquidator incentives and keeper operation.

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Spec: docs/dx_1/003_content_map.md §3 · Depends on: DX-073