Update 'getting started' guide

[jrh3k5]

While trying to get started with Farcaster v2 Frames, I encountered a few issues:

• I didn't realize I wasn't supposed to use the default option for the prompt of import aliasing; I added doc to call that out explicitly.
• The yarn command doesn't automatically add next as an executable; I had to use yarn add... to get it added to the project; I've added doc calling that out explicitly.
• One of the example Demo files is missing an import and function declaration, which prevented compilation; I've added that.
• The app/globals.css file referenced at one point doesn't actually exist in the project by that point, so I've added a note about needing to ensure that file exists.

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PR-Codex overview

This PR enhances the documentation for setting up a NextJS app with frames-v2. It includes additional instructions for project setup, dependency installation, and modifications to the app layout and components.

Detailed summary

• Updated instructions for setting up a NextJS app with frames-v2-demo.
• Added command to install next, react, and react-dom.
• Instructed to create app/globals.css if it doesn't exist.
• Introduced isContextOpen state in the Demo component.
• Added toggleContext function to manage isContextOpen state.

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