docs(plugin-mcp): updates MCP plugin documentation (#15729)

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.

Author: kendelljoseph