docs: better `hljs` support when `navigation.instant` is enabled

[michalsn]

Description This PR adds better support for hljs plugin when navigation.instant is enabled.

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [ ] User guide updated
• [ ] Conforms to style guide

[michalsn]

I noticed this in another plugin: https://squidfunk.github.io/mkdocs-material/reference/math/

And locally it's working just fine. It can be tested more widely after the merge.