doc: navigation

[michalsn]

Description
This PR:

• adds site_url, to make navigation.instant feature work
• fixes the way hljs is initialized to work well with navigation.instant feature
• removes navigation.instant.prefetch feature because it's available only for sponsors of the MkDocs project

Checklist:

• 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

[datamweb]

@michalsn thank you!