Add guide - How to Customize Notifications Based on Tags

[rajlearner17]

Guide purpose
A clear and concise description of what the guide aims to achieve and why it's important.

• Section under /guides/using-guardrails/notifications/
• Setting up email notifications for specific tagged resources.
• E.g. Advanced scenarios: Notifications for different accounts or resources by tags.

Target audience
Specify who this guide is intended for (e.g., beginners, advanced users, specific roles).

Categories
Proposed primary category and categories where this guide would fit best. (e.g., Getting Started, Best Practices, Troubleshooting, Concepts etc).

Outline
A brief outline of the main sections or topics the guide will cover.