Added basic info about developing for web accessibility from W3C (#7740)

* Basic info about developing for web accessibility from W3C

Basic considerations to help you get started developing web content that is more accessible to people with disabilities, from W3C's Web Accessibility Initiative

* Update courses/free-courses-en.md

Thanks

Co-authored-by: Biswadeep Purkayastha <98874428+metabiswadeep@users.noreply.github.com>

* Update courses/free-courses-en.md

Co-authored-by: Eric Hellman <eric@hellman.net>

Co-authored-by: Biswadeep Purkayastha <98874428+metabiswadeep@users.noreply.github.com>
Co-authored-by: Eric Hellman <eric@hellman.net>

courses/free-courses-en.md

@@ -1147,6 +1147,7 @@
 * [Command Line Power User - for web developers](https://commandlinepoweruser.com) - WesBos (email address *required*)
 * [Create a Professional Website with Velo by Wix](https://www.codecademy.com/learn/create-a-professional-website-with-velo-by-wix) - Codecademy
 * [CS50’s Web Programming with Python and JavaScript](https://cs50.harvard.edu/web/) - Brian Yu, David J. Malan (edX Harvard CS50)
+* [Developing for Web Accessibility](https://www.w3.org/WAI/tips/developing/) - World Wide Web Consortium Web Accessibility Initiative
 * [Discover Flask - Full Stack Web Development with Flask](https://github.com/realpython/discover-flask)
 * [Essential Nextjs Typescript Tailwind Stack](https://www.youtube.com/playlist?list=PLKEkvhqFCRwL94nLP-TdIfNmbiZk5FCMx) - Fireship, Ben Awad, Traversy Media, Devs Force
 * [Flask(A Python Microframework) Tutorial](http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world)