DX-075: Write /concepts/funding-and-fees

[IbrahimIjai]

Context

Every cost a trader pays, in one place. Fees scattered across a marketing page
and a tooltip are fees a user discovers at the worst moment; this page is the
complete, honest accounting.

Scope

• Enumerate every cost: open fee, close fee, borrow fee, funding, price impact,
  and network fees. If a cost exists in the contracts, it belongs here.
• For each: how it is calculated, when it is charged, and where it appears in
  the interface.
• Funding: direction, interval, and how the rate is determined, with a worked
  example of a position held across several intervals.
• A single summary table of every fee with its current value, sourced from
  configuration rather than retyped.
• How costs compound over a long-held position.

Acceptance criteria

• No cost charged by the contracts is missing — verified against the fee
  logic, with file paths in the PR.
• The fee table's values come from a generated or referenced source.
• The multi-interval worked example is computed and shown.
• Prose lint passes; no framing of fees as benefits.

Out of scope

• Referral discounts (DX-079).

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Spec: docs/dx_1/003_content_map.md §3 · Depends on: DX-074