docs: add keyless Parallel Search MCP example

[georgeatparallel]

Summary

• Add an optional Parallel Search MCP configuration example to the English and Simplified Chinese guides.
• Keep the existing Stripe example unchanged while documenting a credential-free HTTP endpoint for web search and URL fetching.

Issues

• None.

Verification

• git diff --check
• go test ./docs
• Repository documentation-impact guard
• Credential-free MCP initialize and tools/list, confirming web_search and web_fetch

Documentation impact

Documentation-impact: updated - added a keyless Parallel Search MCP configuration example

Cache impact

Cache-impact: none - documentation-only change; the provider-visible prefix is unchanged
Cache-guard: not applicable - no cache-sensitive paths or runtime behavior changed
System-prompt-review: N/A

Disclosure: I work at Parallel.ai, which operates this MCP endpoint.