UX: Unify `myst start` and `myst build --watch`

[LecrisUT]

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Suppose I want to check both pdf file and the hosted page. With myst build --watch I can actively work on a document since Okular simply auto-reloads when I make changes. On the other hand I have to use myst start to have a localhost:3000 that I can check the pages. It would be nice to have a command to do both.