docs(GlBreadcrumb): Add auto-resize section

What does this MR do?

This update adds documentation for the GlBreadcrumb component's auto-resize feature. It explains that when there's not enough space to display all breadcrumb items in a single row, the component automatically moves items into an ellipsis dropdown (starting with the first item), while keeping the last item (current page) always visible. The documentation emphasizes that for this auto-resize feature to work properly, the component needs to take up all available horizontal space, which requires developers to add appropriate CSS rules depending on their layout context. It includes an example showing how to make the component grow in a flexbox layout using the gl-flex-grow class.