docs(features): add Gas Top-Up intro

[scout-relayprotocol]

Implements MAR-1414: Gas Top-Up feature page intro

What this changes

• Adds the approved payout-focused marketing intro to features/gas-top-up.mdx.
• Covers how Gas Top-Up works, why it matters, target audiences, and current limitations.
• Preserves the existing enablement and support-checking guidance below the new intro.
• Links related Gasless Execution, Price Stabilization, and Supported Tokens & Routes docs.

How I verified

• git diff --check
• Confirmed all added internal link targets and the fixed-rates anchor exist.
• Manually reviewed the final MDX against the repository style guide and approved Notion copy.
• No package scripts are defined in this docs repository; GitHub handles PR-time review.

Open questions

• None. The existing technical sections remain intentionally unchanged in scope, aside from parameter emphasis matching the style guide.

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🤖 Opened by Scout (autonomous). Ready for human review.
Linear: https://linear.app/relayprotocol/issue/MAR-1414/gas-top-up-feature-page-intro

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