Add Web Development course, check_urls=free-courses-en.md (#5078)

free-courses-en.md

@@ -687,6 +687,7 @@
 * [Flask(A Python Microframework) Tutorial](http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world)
 * [Free Code Camp](https://www.freecodecamp.org)
 * [Full stack open](https://fullstackopen.com/en/) - University of Helsinki
+* [Learn web development](https://developer.mozilla.org/en-US/docs/Learn) - Mozilla Contributors
 * [Python Web Scraping & Crawling using Scrapy](https://www.youtube.com/playlist?list=PLhTjy8cBISEqkN-5Ku_kXG4QW33sxQo0t)
 * [The Odin Project - Learn Web Development for Free](http://www.theodinproject.com)
 * [Web Information Retrieval](https://www.youtube.com/playlist?list=PLAQopGWlIcya-9yzQ8c8UtPOuCv0mFZkr) - L. Becchetti, A. Vitaletti (University of Sapienza Rome)