fix(sidebar): render nested subpages as details

[LeoMcA]

Relates to: #642

Currently pages defined in a sidebar with:

type: listSubPages
depth: 2
nested: true

Render as a nested list, but with no details elements. This results in some weird rendering:

e.g. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import.meta

https://developer.mozilla.org/en-US/docs/Glossary/Boolean

With this change, those render to <details> elements, like so:

and:

This is particularly important for the sidebar changes proposed in #642:

Without this change:

With this change:

[github-actions]

23dacd5 was deployed to: https://rari-pr654.review.mdn.allizom.net/

[Rumyra]

lgtm @LeoMcA

[caugner]

Hm, this is weird:

Took: 17.039215321s - 0 changes in 0 of 14380 files (0.0%)

[caugner]

Initial pass. Is there a chance to add a unit test?

[caugner]

nit: could this be a separate function with a self-documenting name like write_li_with_details?