Source for admonition/ callout

Question

Not sure if this counts as a ``high quality post" but I'd appreciate any advice on the LaTeX template/ package source that I can create a callout with:

Here's an image of the desired output:

Answer 1

With tcolorbox:

\documentclass{article}
\usepackage{lipsum}
\usepackage[most]{tcolorbox}
\tcbuselibrary{skins,breakable}
\newtcolorbox{mybox}[2][]{breakable,sharp corners, skin=enhancedmiddle jigsaw,parbox=false,
boxrule=0mm,leftrule=2mm,boxsep=0mm,arc=0mm,outer arc=0mm,attach title to upper,
after title={.\ }, coltitle=black,colback=gray!10,colframe=black, title={#2},
fonttitle=\bfseries,#1}
\begin{document}
\lipsum[2]
\begin{mybox}{The Bonnet rotation}
The helicoid, $\boldsymbol{\varphi}$, and the catenoid, $\widetilde{\boldsymbol{\varphi}}$ 
are related through the Bonnet rotation, $\boldsymbol{\sigma}_\theta$, which is the 
weighted sum of the two minimal surfaces:
\[
\boldsymbol{\sigma}_\theta=(\cos \theta) \boldsymbol{\varphi}+
(\sin \theta) \tilde{\boldsymbol{\varphi}}
\]
In a Bonnet rotation every surface element maintains its normal vector but rotates 
a given angle in its tangent plane. If and only if the surface is a minimal surface, 
then the surface elements all fit together again. The Bonnet rotation is 
an isometry of the surface.
\end{mybox}
\lipsum[6]
\end{document}