docs: add optional Parallel Search MCP example

[georgeatparallel]

Why

AgentCrew supports remote MCP servers, so users can opt into Parallel Search through the existing mcp_servers.json extension point without changing the built-in web search provider or defaults.

Changes

• Document an optional Parallel Search remote MCP server scoped to the example Researcher agent.
• Note that users must add it to their own configuration, no API key is required, and only selected search queries and fetched URLs are sent to Parallel.
• Leave active configuration, Tavily, defaults, runtime, and dependencies unchanged.

Validation

• Parsed and asserted the owned Markdown JSON example with Python in the isolated runner.
• git diff --check

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