feat: support rendering the current section

[innocenzi]

When using this package with Inertia, I often need the current section to display data related to the current page. Unfortunately, this is quite hard to achieve - I need to create a custom renderer in my application code.

This PR solves that problem by adding a new SectionRenderer and a current method on Navigation, in order to render the section that is currently active.

Additionally, I updated the docs to add an example of this feature, and updated the previous examples to use Navigation::make() instead of app(Navigation::class).

[freekmurze]

Could you take a look at this @sebastiandedeyne ?

[innocenzi]

Sorry to bother - may I kindly ask for this PR to be reviewed/merged? 😊

[sebastiandedeyne]

Looks good, thanks!