Consider using +++ as the panel delimiter

[choldgraf]

I was just thinking that the behavior in shpinx-panel is in some ways similar to that of MyST-nb notebooks. To that extent, what do you think about using the same syntax to define delimiters (+++)? Maybe that could reduce the general cognitive burden for users across these packages as +++ could be the generally accepted syntax for “a break between sections of your content”

Note that I’m just talking about the + symbol, not the approach to metadata...though now I wonder if we should revisit that on the MyST-nb side?