docs: improve getting started chapters

[shahednasser]

As more users are getting confused on the steps that follow create-medusa-app, or how to troubleshoot common issues, it's become clear we need a separate installation chapter rather than relying on the introduction.

Changes in this PR:

1. Add an Installation chapter after the introduction chapter. This chapter instructs the user on how to install Medusa with create-medusa-app, how to troubleshoot common errors, how to start the Medusa development server, and more useful details.
2. Move first customization to the introduction chapters
3. small fixes to CORS troubleshooting

Closes DX-1047

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