docs(x402): update explorer listing guide for registration/ownership split

[nolnol3]

Updates /x402/explorer/sell to match the current Explorer flow:

• Listing and ownership are now separate. Registering puts a service in the catalog; proving you hold the payout address is its own one-time signature (Verify ownership), done at registration when you sign with the payout wallet, or any time later.
• Check before signing. Endpoints are probed live before the signature — the form shows the real terms, the payout address, and the endpoint's own refusal text; endpoints that validate input first accept a pasted sample request.
• One batch signature registers every endpoint of a service; the wallet statement shows what is approved.
• Registering with a non-payee wallet now creates an unverified listing (manageable only by the payee) instead of being rejected.
• Verified ownership gates editing, withdrawal, and ERC-8004 identity minting; approval still gates public visibility.

Removed two screenshots of the previous single-step form. Build (with link check) passes.