docs: rewrite What is Relay page (MAR-1412)

[scout-relayprotocol]

Implements MAR-1412: What is Relay docs page rewrite

What this changes

• Rewrites what-is-relay.mdx around Relay's onchain payments positioning.
• Updates scale metrics to 85+ chains, 186M transactions, and $40B+ volume.
• Removes stale stats, banned bridge-led positioning, unconfirmed partner claims, and absolute reliability guarantees.
• Preserves the existing use-case cards and documentation links.

How I verified

• Validated docs.json as JSON.
• Checked MDX frontmatter and balanced Columns/Card component pairs.
• Confirmed the existing use-case and documentation links remain present.
• Scanned for stale or unresolved claims and ran git diff --check.
• This repository does not define a local package-based lint or test suite; deployment validation is left to the docs preview/review flow.

Open questions

• The proposed tagline remains omitted because it is still marked TBC.
• Alchemy, Conduit, Caldera, and OneBalance remain omitted pending partner confirmation.
• The 99.9%+ transaction-success figure remains omitted pending data sign-off.

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🤖 Opened by Scout (autonomous). Ready for human review.
Linear: https://linear.app/relayprotocol/issue/MAR-1412/what-is-relay-docs-page-rewrite

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