ENH: update show code cell prompt to be more concise

[mmcky]

This PR uses myst-nb config to update the Show code cell source prompt to Show source

The source is backed into the myst-nb node so we could override through our own extension if needed

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[mmcky]

@jstac would you be happy enough with Show source?

We can override the myst-nb text via _config.yml that is responsible for writing these objects in html which is a pretty simple fix but the {type} is hard coded into the package so Show {type} will produce Show source, Show output and Show contents based on the node type that is being parsed.

The alternative would be to override this node in myst-nb via a sphinx extension which can be done in quantecon-book-theme -- it is just a bit more involved.

[jstac]

Sure @mmcky , thanks