Incorporate feedback from UX sessions into the 12-factor tutorials.

Summary of changes

• cat rockcraft.yaml and update the architecture
• when updating the version, show a snippet of rockcraft.yaml to make it explicit
• Intro to the initial Rockcraft section explaining a rock better
• Make Multipass much more mandatory in the language
• More explanation in the setup section – copied over the text about Rockcraft
• Add the $(dpkg --print-architecture)where applicable to make the commands accessible to all architectures
• Emphasize-lines in code blocks where appropriate (didn’t see anything)
• Moved the LXD/Docker warning to the first pack attempt, and added another potential clue in the output
• Improved product terminology (removed code formatting for npm and go instances, converted some of the profile mentions to plain text, updated the Spring Boot tutorial title in the navigational menu)

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