Call-outs in articles

[trjordan]

Due to the complexity of a lot of what we're talking about (or maybe just our writing style), many of the articles have asides or avenues for further explanation. It would help to move these asides out a call-out box, so they're not inline with the straight-line explanation.

We'd probably need code support for this, unless there's a Markdown feature I don't know about.

Examples:

• In On Your Laptop, we introduce the concept of clusters, listeners, and listener filters. These aren't obvious definitions, but defining them inline (which we currently do, in a clipped sort of way) feels like a distraction. It'd be great to pull that definition out.
• Routing and Service Discovery introduce the concept of a control plane. It's a whole topic with a great post covering it. Calling it out as further reading seems good.