Recently Google Classroom came out with the ability for websites to have a share to Google Classroom button. One of my favorite examples is Flat (https://flat.io/en) which allows for collaborative music composition online. With the click of a button teachers can assign Flat to their music students right to the Google Classroom stream. Students can submit their work directly from Flat to Google Classroom.
Students are able to use the Share to Classroom button to submit work to Google Classroom. Students click on the button, choose the class they are submitting work to and then which assignment they want to submit to.
For websites that do not natively include the share to Google Classroom button, there is a Chrome extension (not made by Google, made by @MrCaffrey) that allows teachers to assign webpages directly to Google Classroom and students to submit their work. . Click on the extension, choose your class you want to assign the webpage to and create your assignment. This will be published to the stream in Google Classroom.