Docs: validate and document the pricing changes for GA

[nijoe1]

Validate the new per-operation pricing and reflect it in the GA docs.

• Lead with a simple rates-and-fees table; tuck the mechanics into a separate section.
• Keep three terms distinct: rates (recurring storage + proving), fees (small per-operation charges), lockups (new ~0.1 USDFC fixed reserve, drawn down as fees are paid and topped up as needed).
• Per-operation fees are tiny, like gas: lead with headline rates, footnote the detail.
• For scaling addPieces (base + per-piece multiplier) with unknown batch size, assume the single-piece price.

References: FilOzone/filecoin-services#469 (pricing), FilOzone/filecoin-services#488 (implementation).