docs: make keyless verb lists surface-aware

[Max17190]

Why

A single keyless verb list misleads agents across surfaces: CLI/SDK/REST keyless includes interact, but hosted MCP keyless does not. ai-onboarding still used fallback framing, and rate-limits Keyless still lumped MCP with CLI/SDK as if interact were keyless everywhere.

Summary

• Replace the Get credentials keyless paragraph in ai-onboarding.mdx
• Drop “meant as a fallback” / human-unavailable framing
• Make keyless verb lists surface-aware in ai-onboarding.mdx and rate-limits.mdx:
  • CLI / SDKs / REST: search, scrape, interact, and parse
  • Hosted MCP: search, scrape, and parse (interact needs an API key)
• Keep the signup nudge for higher rate limits, other endpoints, or production

Aligns with:

• Document keyless scrape/search/parse in MCP instructions firecrawl-mcp-server#307
• https://github.com/firecrawl/firecrawl-web/pull/2766

Test Plan

• Diff is only base ai-onboarding.mdx and rate-limits.mdx (no localized files)
• ai-onboarding Get credentials distinguishes CLI/SDK/REST from hosted MCP
• rate-limits Keyless distinguishes CLI/SDK/REST from hosted MCP
• Both pages include parse on the correct surfaces and note interact needs a key on hosted MCP
• Links to /rate-limits#keyless-no-api-key and /mcp-server resolve
• Preview Get credentials and Keyless sections on the docs site

[hmishra2250]

Make this surface-aware: it's accurate for one path, misleading for the other. CLI/SDK/REST keyless = scrape, search, interact, parse; hosted MCP keyless = scrape, search, parse (interact needs a key on MCP). Current line drops parse and over-includes interact.