Added Douglas schmidt cpp screencast (#7389)

* Added Douglas schmidt cpp screencast

* Updated free-podcasts-screenshots.md

* Update casts/free-podcasts-screencasts-en.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

* Update free-podcasts-screencasts-en.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

casts/free-podcasts-screencasts-en.md

@@ -53,6 +53,7 @@
 * [C++ Programming Video Lectures](https://www.youtube.com/playlist?list=PLTZbNwgO5ebo64D1k0DJQGX30X6iSTmRr) - Saurabh School of Computing (screencast)
 * [C++ Series](https://www.youtube.com/playlist?list=PLlrATfBNZ98dudnM48yfGUldqGD0S4FFb) - The Cherno (screencast)
 * [C++ Standard Library](https://www.youtube.com/playlist?list=PL5jc9xFGsL8G3y3ywuFSvOuNm3GjBwdkb) - Bo Qian (screencast)
+* [C++ STL by example](https://www.youtube.com/playlist?list=PLZ9NgFYEMxp5oH3mrr4IlFBn03rjS-gN1) - Douglas Schmidt (screencast)
 * [CppCast](http://cppcast.com) - Conor Hoekstra, Jason Turner, JeanHeyd Meneide, Matt Godbolt, Rob Irving (podcast)