Docs: Update the "Prompts" page

[leventebalogh]

What changed?

The following titles in the left sidebar are a bit confusing (similar titles referring to different things):

• Reference
  • CLI commands (available NPM scripts for development)
  • Prompts (prompts for the create-plugin command)

This PR mostly

• Removes the docs for the NPM scripts (I think this is kind of unnecessary)
• What's currently "Prompts" becomes "CLI commands"
• Moves the update command docs from "Migration" to "Reference > CLI commands"

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[josmperez]

@leventebalogh documenting all the CLIs available for a tool is pretty customary, whether it's in a markdown or in a "help" command available from the tool itself. i'm not sure it's necessary to remove the existing list of commands, but let's do it if there's a general feeling that it's information overload.

[josmperez]

I know this PR is merged, but I just want to say I appreciate your contribution to these docs. Thanks!