feat(docs): More info on the email dev command

[gabrielmfern]

This adds currently three sections to the email dev portion of the /cli page we have on our docs:

• "Where can I place my static files for previewing"
• "How to make the preview server ignore directories"
• "How can I define props specific to the email's preview"
• "The heuristics for files to be considered emails"

I believe these are very important as more often than not users will ask about these
and want to know it, and having something on the docs that they can search for and find
is a much better experience than having to wait our response.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
react-email	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 18, 2024 4:40pm

1 Ignored Deployment

Name	Status	Preview	Comments	Updated (UTC)
react-email-demo	⬜️ Ignored (Inspect)	Visit Preview		Mar 18, 2024 4:40pm