Add custom playground guide

[dalance]

Custom playground for another language can be achieved through theme customization, but the customization points are difficult and not documented.

This document gathers the existing workaround at #1453 and my trial. In my environment (https://dalance.github.io/veryl/book/), the way described in this document achieved editable, syntax highlighted playground for my language.

I think some points seem to be hacky (ex. editor.js replacing). It may be able to be simplified by mdBook's official support.

Related: #350 #1759

[ghost]

This guide is helpful! Thank you!