Add a helpful message box to the CLI when there is a new theme version available

[choldgraf]

I regularly get burned because the documentation for mystmd relates to a newer theme than whatever exists in the _build folder. It would be helpful if there were a way to really clearly tell users "you're using an outdated theme, here's how to upgrade".

The message that mystmd itself sends is quite helpful and noticeable, would it be possible to check for new theme versions and surface a similar message?

For example: "New theme version detected. Run myst clean to theme cache."

[rowanc1]

@fwkoch wrote a bit about this here:

• Theme versioning improvements! #854