Add visual indicator for active page in navigation menu

When using the Dioxuslabs documentation at https://dioxuslabs.com/learn/, there is no visual indicator in the navigation bar showing which page is currently active.

Although the breadcrumbs at the top of the documentation pages reflect the navigation bar's naming convention and hierarchy, they are not always visible. This is the case, for example, if you scroll down or when the menu is opened in a small window or on a small screen, such as on a phone. In these cases, the navigation menu takes over the entire screen and the breadcrumbs are hidden. Having a visual indicator in the menu would be helpful for navigation in these cases.

Additionally, it is not always easy to match the most prominent text element and navigation title of a page with each other. For instance, when you open the ["Adding State"](https://dioxuslabs.com/learn/0.6/guide/state#) link from the navigation bar, the page's most prominent text element is "Interactivity". That "Adding State" and "Interactivity" describe the same page can be confusing, especially for new users. While harmonizing the navigation title with the most prominent text element can be helpful, adding a visual cue to the navigation menu itself would also be helpful in cases where a page's full title needs to be shortened to fit in the navigation menu.

The entry point is the documentation site at dioxuslabs.com/learn/; inspect how its navigation menu determines the current page, including the small-screen menu. Done means the active page has a visible cue in the navigation and remains usable when breadcrumbs are hidden.

Written by the indexing model from the issue text.