chore(dev.md): remove old instructions from dev.md

[SG-SWE073]

Removed outdated instructions:

• "run setup" is no longer a command in the node package
• localhost:8000/demo no longer exists
• /site is no longer in the project's directory

[slmjkdbtl]

Thank you!

[SG-SWE073]

You're welcome :) . I'll be doing some other doc related updates soon.