Add custom playground guide for v0.5

[dalance]

This PR documents how to implement custom playgrounds.

This is a successor to PR #2286.
The method described in #2286 is no longer available in v0.5, so it has been rewritten for v0.5, and
it was possible to implement it with fewer patches than the previous version.

Related: #350 #1453 #1759

[GuillaumeGomez]

Sorry for the delay, we now have an mdbook team so things should move a bit faster here. This sounds like a very good idea to add an example! Do you mind adding a GUI test to ensure that the code you provided won't become outdated when we make updates?

[GuillaumeGomez]

Need a GUI test using the JS code example to ensure it doesn't become outdated.

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[dalance]

Thank you for your review.
I'm glad the mdBook team has been established.

When I wrote this documentation, I understood that review bandwidth was extremely limited,
so I wrote a procedure which can be implemented without making changes to mdBook itself.

If minor changes are acceptable, I can clean up some of the JavaScript hacks, what do you think?
For example, if we slightly adjusted the condition at

mdBook/crates/mdbook-html/src/html/tree.rs

Line 1010 in 74b14bb

if !is_playground {

,
the section titled "Restrict the .playground selector to <pre>" will no longer be necessary.

[GuillaumeGomez]

If that can make things simpler, then it's worth a try!

[dalance]

I wrote 2 issues to improve custome playground support.
If both of them are accepted, the way of this PR is significantly simplified because theme override is not necessary.
Especially it is important that book.js override is not necessary because it causes manual synchronization to mdBook update.

[dalance]

@GuillaumeGomez
I created #3218.

Regarding #3216, since it would have a bit large impact, such as requiring the addition of a configuration value,
I'd like to prepare it only after reaching a certain level of consensus.
What do you think?

If there is no prospect of it being accepted in the short term, I am also open to rewriting this guide on the assumption that #3216 can't be used.