docs: replace stale README transaction example guidance

[TisanSmart232]

Closes #1669

Summary

• replace the stale low-level transaction executor example at the end of the README
• keep the stable GraphQL endpoint and generated schema path references
• point application developers to the Fuel TypeScript SDK, Fuels CLI, and GraphQL docs instead of asking them to hand-craft hex transaction payloads

This follows the maintainer guidance in #1669 to remove the broken example and direct readers toward the SDK/tooling for application workflows.

Validation

• git diff --check
• confirmed the added Fuel docs links return HTTP 200

Docs-only change; no Rust tests were run.

[fuel-cla-bot]

Thanks for the contribution! Before we can merge this, we need @TisanSmart232 to sign the Fuel Labs Contributor License Agreement.

[cursor]

PR Summary

Low Risk
Documentation-only README edit with no code or runtime behavior changes.

Overview
Replaces the outdated GraphQL service / transaction executor section with Client APIs, keeping /v1/graphql and crates/client/assets/schema.sdl as the stable references.

Removes guidance on instant block production, RocksDB persistence, and hand-submitting hex-encoded submit mutations. Application developers are directed to the Fuel TypeScript SDK, Fuels CLI, and Fuel GraphQL docs instead of low-level transaction crafting.

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