Improved grammar articles navigation

In this update, I've improved the navigation experience for grammar articles, making it easier to find and access the content you're looking for.

So far it was difficult to distinguish an article from a folder in the sidebar menu on desktop. Sometimes it was possible to open a folder and it contained some content, some folders were not navigable and only expanded their subtree. Now all folders are not navigable and all content has been moved to articles. It's much easier now to choose the article you're interested in without the need to navigate through many folders.

On the other hand, all breadcrumbs in grammar articles are navigable now, even folders. After you click on a folder in the breadcrumb, you will see a subtree of its articles. This will be especially helpful on mobile devices, because there you don't see the sidebar menu as on desktop.

Some links in the articles tree contain redirects to different parts of the tree. I've added a clear icon and tooltip to make these redirects more obvious.