feat: update README for improved clarity on MCP client configuration …

[freema]

This pull request significantly improves the README.md documentation for the MCP Google Sheets server, making it easier for users to get started and configure the integration with various MCP clients. The changes reorganize the instructions, clarify setup steps, and add support details for multiple clients.

Documentation improvements and expanded client support:

• Added a "Key Features" section highlighting advanced operations, flexible authentication, and production readiness.
• Reorganized setup instructions to provide clear, client-specific configuration steps for Claude Code, Claude Desktop, Cursor, Cline, and other MCP clients. [1] [2]
• Updated all configuration examples to use npx mcp-gsheets@latest for easier installation and automatic updates, replacing previous node/local build instructions. [1] [2]
• Added instructions for alternative authentication using a JSON string, making the setup more flexible for CI/CD and containerized environments.
• Provided a dedicated section for local development, including interactive and manual setup instructions for contributors.…and requirements