Add callout type name to all callouts above the callout title for clarity

[jfrost-mo]

Its unclear to somebody unfamiliar with the carpentries workbench what the symbols in the callouts mean. We propose adding a text label just above the custom callout title, in a similar fashion to GitHub's admonitions:

[astroDimitrios]

Or perhaps easier to just add the callout type name before the custom callout title turning something like:

What are branches?

into

Callout: What are branches?

[jfrost-mo]

For example: