adding clarity around install instructions

[jeremyzilar]

I am adding a few changes to make the install instructions a little more clear.

Previously, it seemed like you needed to run npm create sanity@latest init --env from both the /app and /studio folders, when it looks like you only need to run that command from the /studio folder.

It was also not clear where to run npm run dev, so I indicated that it should also be ran from both folders.

Hope this is helpful.

[vercel[bot]]

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[kmelve]

We have now updated this template and fixed these issues. Thanks for the PR, though!