How to make StackExchange-esque quotation block in LaTeX?

Question

I mean this one:

looks quite pretty. Optionally with a background.

Thanks in advance

Edit This may be a good way to start:

\documentclass[preview]{standalone}
\usepackage[most]{tcolorbox}
\newtcolorbox{mbox0}{standard jigsaw, opacityback=0.2, colback=gray, bicolor, frame hidden, arc=4pt}
\begin{document}
\begin{mbox0}
\begin{tabular}{|p{12pt}}
Prolog
\end{tabular}
\end{mbox0}
\end{document}

Now the left bar just needs to be thicker and round

Answer 1

The following should get you started:

\documentclass[preview, border=20pt]{standalone}
\usepackage[most]{tcolorbox}
\newtcolorbox{mbox0}{enhanced, 
                     colback=white,
                     frame hidden,
                     boxrule=0pt,
                     borderline west={3pt}{0pt}{gray!50, line cap=round}}
\begin{document}
\begin{mbox0}
Prolog
\end{mbox0}
\end{document}