docs: update Gelato Relay integration for @gelatocloud/gasless migration

[donoso-eth]

What it solves

Update docs to reflect the migration from @gelatonetwork/relay-sdk to @gelatocloud/gasless in the relay-kit. SyncFee is removed, apiKey is now required, and executeTransaction returns a task ID string directly.

Related SDK PR: safe-global/safe-core-sdk#1305
Gelato docs: https://docs.gelato.cloud/gasless-with-relay/gelato-turbo-relayer/overview

Changelog

• Update Gelato Relay guide to reflect migration from @gelatonetwork/relay-sdk to @gelatocloud/gasless
• Remove SyncFee section (no longer supported), only sponsored transactions via 1Balance remain
• Update code examples: apiKey is now required, options/isSponsored removed, executeTransaction returns string directly
• Add getTaskStatus() usage and Gelato RPC polling for transaction status
• Update all docs.gelato.network links to docs.gelato.cloud
• Update Relay Kit descriptions in overview and relay-kit pages

Checklist

• I've followed all safe-docs pull request rules
  Link to https://safe-global.notion.site/safe-docs-pr-rules is broken

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Note

Low Risk
Docs-only changes that update third-party links and usage examples; low risk aside from potentially confusing developers if the new Gelato API/return types are misdocumented.

Overview
Updates Relay Kit documentation to reflect Gelato’s migration to docs.gelato.cloud and the 1Balance-sponsored-only model.

Rewrites the Gelato integration guide to require an API key, remove SyncFee/callWithSyncFee and isSponsored options, and update examples to match the new Relay Kit API (createTransaction/executeTransaction returning a task ID string, getTaskStatus() status checks, and optional direct Gelato RPC polling). Also refreshes Relay Kit descriptions in the SDK overview and Relay Kit landing page, plus the glossary relayer link.

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