[plan] Add "What is GitHub Agentic Workflows?" section to homepage #6506

Objective

Add a clear, prominent explanation of what GitHub Agentic Workflows (gh-aw) is at the very top of the homepage, before diving into concepts like "agentic workflows."

Problem

The current homepage jumps directly into explaining "What are Agentic Workflows?" without first establishing what the tool itself is. New users are confused about whether gh-aw is a GitHub feature, a CLI extension, a framework, or a service.

Approach

Add a new section at the top of /docs/src/content/docs/index.mdx (before "What are Agentic Workflows?"):

## What is GitHub Agentic Workflows?

GitHub Agentic Workflows (gh-aw) is a GitHub CLI extension that lets you write AI-powered automation workflows in plain English using markdown files. These workflows run on GitHub Actions and can make intelligent decisions about your repository.

**Key features:**
- Write workflows in natural language markdown
- AI agents execute tasks with reasoning and decision-making
- Runs securely on GitHub Actions infrastructure
- Built-in safety controls and pre-approved actions

Files to Modify

• docs/src/content/docs/index.mdx - Add new section at the top

Acceptance Criteria

• New section appears at the very top of the homepage
• Clearly states gh-aw is a GitHub CLI extension
• Explains in simple terms what it does (AI-powered workflows in markdown)
• Mentions it runs on GitHub Actions
• Uses beginner-friendly language without jargon

Reference

From noob test report: "Missing 'What is this?' on Homepage" - Critical Issue #1
Related to #6505

AI generated by Plan Command for discussion #6502