DX-073: Write /concepts/margin-and-leverage

[IbrahimIjai]

Context

Where most trading losses begin. The page must make the relationship between
collateral, position size, and effective leverage concrete enough that a reader
can compute their own numbers before opening a position rather than after.

Scope

• Definitions: collateral, position size, effective leverage, initial margin,
  and how the interface displays each.
• The formula relating them, with a worked example and the arithmetic shown.
• How adding or removing collateral changes leverage on an open position.
• Which collateral assets are supported, sourced from the code.
• What the maximum leverage is and where that limit is enforced.

Acceptance criteria

• Formulae match the contract implementation — name the file and function.
• At least two worked examples with computed numbers, including a losing one.
• Supported collateral matches the code, not a written list that will rot.
• Cross-linked to liquidation and fees.

Out of scope

• Liquidation mechanics (DX-074).

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Spec: docs/dx_1/003_content_map.md §3 · Depends on: DX-072