docs: add table of contents to copilot instructions

Add comprehensive table of contents to .github/copilot-instructions.md
for improved navigation through the contributor guide.

The TOC includes:
- Project overview and repository structure
- Development workflow and command reference
- Conventions, standards, and testing architecture
- Getting started guides for different user types
- AI assistant guidelines and restrictions

Benefits:
- Improves navigation for this comprehensive guide
- Makes the document more accessible to contributors
- Maintains visual consistency with emoji headings
- Enhances professional appearance of documentation

Author: josecelano

.github/copilot-instructions.md

@@ -1,5 +1,26 @@
 # Torrust Tracker Demo - Contributor Guide
 
+## Table of Contents
+
+- [🎯 Project Overview](#-project-overview)
+  - [Current Major Initiative](#current-major-initiative)
+- [📁 Repository Structure](#-repository-structure)
+  - [Key Components](#key-components)
+- [🛠️ Development Workflow](#️-development-workflow)
+  - [Quick Start for Contributors](#quick-start-for-contributors)
+  - [Main Commands](#main-commands)
+- [📋 Conventions and Standards](#-conventions-and-standards)
+  - [Twelve-Factor App Principles](#twelve-factor-app-principles)
+  - [Git Workflow](#git-workflow)
+  - [Code Quality Standards](#code-quality-standards)
+  - [Testing Requirements](#testing-requirements)
+  - [Security Guidelines](#security-guidelines)
+- [🚀 Getting Started](#-getting-started)
+  - [For New Contributors](#for-new-contributors)
+  - [For Infrastructure Changes](#for-infrastructure-changes)
+  - [For AI Assistants](#for-ai-assistants)
+- [📖 Additional Resources](#-additional-resources)
+
 ## 🎯 Project Overview
 
 **Torrust Tracker Demo** is the complete production deployment configuration for running a live [Torrust Tracker](https://github.com/torrust/torrust-tracker) instance. This repository provides: