Improve documentation for breadcrumbs #1806

The documentation for rendering breadcrumbs with Eleventy Navigation (here) would be more helpful if it showed the most common use case, rendering breadcrumbs for the current page. This would help new users to get started quickly and also help them avoid the pitfall of attempting to load breadcrumb info on a page without a navigation key (which causes an unclear error of Node does not exist: undefined).

E.g. for Liquid:

{% if eleventyNavigation.key %}
    {{ collections.all | eleventyNavigationBreadcrumb: eleventyNavigation.key | eleventyNavigationToHtml }}
{% endif %}