DX-076: Write /concepts/order-types

[IbrahimIjai]

Context

The interface offers market, limit, and trigger orders. Each carries a different
execution guarantee, and the difference between "this will fill" and "this may
fill" is exactly what a reader needs to understand before relying on a stop.

Scope

• Each order type: what it does, when it executes, and what is not guaranteed.
• Trigger orders specifically: take-profit and stop-loss, how trigger price
  relates to execution price, and the conditions under which a stop does not
  protect a position.
• Order lifecycle: created, pending, executed, cancelled, expired — matching the
  states the interface and indexer actually use.
• Slippage and how the slippage setting interacts with each type.
• How each maps to contract calls, linking to the reference pages.

Acceptance criteria

• Order states match the indexer and apps/web implementations, with file
  paths named.
• The "what is not guaranteed" content is explicit for every type.
• The trigger-order section describes the real gap risk without hedging.
• Cross-linked to the trading guide and liquidation page.

Out of scope

• The trade-panel walkthrough (DX-077 is oracles; the guide is DX-077+).

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Spec: docs/dx_1/003_content_map.md §3 · Depends on: DX-075