Update README to enhance CLI and VS Code Extension sections

[cassiebreviu]

https://github.com/cassiebreviu/spec-kit/tree/sdd-vscode-ext

[Copilot]

Pull Request Overview

This PR enhances the README by splitting the "Get started" section into two distinct paths - CLI and VS Code Extension - to better guide users based on their preferred development environment.

• Restructured table of contents to separate CLI and VS Code Extension getting started sections
• Added comprehensive VS Code Extension installation and setup instructions
• Updated section headings to clarify the distinction between CLI and extension workflows

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 2 changed files in this pull request and generated 2 comments.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 3 changed files in this pull request and generated 1 comment.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 3 changed files in this pull request and generated no new comments.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}