Integrating REPLs in the guide

[mindrones]

Related to #107, there have been talks about integrating the REPLs in the documentation.

As mentioned by @pngwn in #382 this could hit performances if we embed tenths of these in a long guide so maybe an option could be to make a REPL run only after clicking on it.

Related to REPL searchability, we could mark that a certain example is embedded in the official guide.

There should be way to avoid the scroll-trap to avoid what happens on maps embedded in scrollable documents (again maybe after mouse-entering/clicking on it?).

It should be possible to fold/minimize the repl to facilitate reading the text around it in a guide.

[pngwn]

Regarding performance, you could just not render the REPL if it isn't in view/ about to come into view. The same could be true of other interactive elements on the page. This is easier said than done obviously, but it is a possibility.

[mindrones]

A possible application (mentioned in the FAQ issue linked above) are how tos: for each use case "How do I ...X" we could show an example in an embedded REPL, so that the user don't have to leave the documentation.