refactor(unstable): remove next edit suggestions

[benbrandt]

Follow-up to #2264, which moved NES to To be removed. This PR targets main and completes the removal.

• Remove the unstable_nes Cargo feature and the NES implementation from ACP v1 and v2: types, capability fields/builders, method names, requests/responses, notifications, and dispatch arms.
• Remove the NES-only document-event notifications and position-encoding negotiation.
• Regenerate both draft schemas, method metadata, and API reference docs, removing 57 definitions from each schema.
• Delete the retired RFD and navigation entry, add a Removed lifecycle update, and point historical updates to that notice instead of the deleted page.
• Update v2 guides while preserving optional session capabilities and extension-method passthrough.