I am the administrator and content editor of a Plone 5.0 site. How can I learn that a user has submitted content for publication, so that I am able to have a look at this content in time? So far, I have not found any notification mechanism.
You could use a content rule
You could use a collection that is visible to (only) you that just shows 'state: submitted for publication'
(or you could use a portlet)
Review list portlet. It will only be visible to users with the review role. There is also a
full_review_list view (which is linked from the portlet), which gives you a list of all current items in review. You can access it on any content object, e.g. http://your-plone-site.com/full_review_list.
For a notification (e.g. via email), use a content rule as @espenmn mentioned.
I've created content rules that email those who can review content, and I've added the review portlet to the front page of the site.
You can also add the review portlet to the dashboard, if you have been training your users to use the dashboard.
(plug: you can configure Castle CMS so that by default logged in users get taken to their dashboard)