Feature: highlight active page in `NavigationBar`

[pd95]

This Pull Request implements highlighting the currently active page in the NavigationBar as illustrated with the image below. It supports Links and DropDowns. The implementation relies on Bootstraps "active" CSS class for "nav-link" elements.

Current issues:

• According to the documentation (see link above) one should add also "aria-current: true" attribute to the currently selected element. To be implemented
• "On-by-Default": the feature cannot be disabled/enabled. There are no configuration flags. To be discussed: Do we need configuration for this feature? If yes: wouldn't most users expect this feature to be active by default? This would cause regressions for existing users.
• The current implementation works, but is probably not nice: I had to add a new property currentRenderingPath to PublishingContext and set it accordingly in some render methods to allow checking for it in the NavigationBar implementation so we know which link should be marked as "active". I think this would be something to be added to an @Environment.

[twostraws]

This was something I had hoped to get into the initial release, but it just didn't make the cut because I ran out of time, so I have no problem merging it in now – I don't think it will cause any problems. You've also solved it exactly the right way given the current code structure, so I think we should get this merged in. Are you happy to move forward?

[twostraws]

Thank you, @PetrPrazak and @pd95! 🙌 All good to merge now?

[pd95]

👍🏻 from my side.