docs(plugin-mcp): updates MCP plugin documentation

[kendelljoseph]

This PR expands the MCP plugin docs to cover all current features, patterns, and workflows.

New sections added

• How Access Control Works — step-by-step explanation that plugin config and API Key permissions are two separate gates.
• Globals — documents find/update tool generation for singleton globals.
• Reducing Token Usage with select — explains the select JSON string parameter.
• Modifying Responses — documents the overrideResponse hook on collections and globals.
• Localization — covers the automatic locale and fallbackLocale parameters added when localization is enabled in Payload config.
• Tracking MCP Events — documents the onEvent callback in mcp.handlerOptions.
• Virtual Fields — explains that computed/read-only virtual fields are excluded from create/update schemas but still appear in find responses.
• Testing Your MCP Endpoint — covers npx @modelcontextprotocol/inspector and direct curl testing workflows.
• Claude Code Connector — claude mcp add command snippet for connecting Claude Code as an MCP client

Improvements to existing sections

• Performance — expanded from a single description tip to four concrete levers: strong descriptions, select, overrideResponse, and least-privilege operation enablement.

[DanRibbens]

Similar to this optimization, what about depth?
Is the default depth 0 or should that be documented too?