Added a screencasts section to the docs that will link to screencasts on the YouTube channel.

docs/source/screencasts.rst

@@ -0,0 +1,12 @@
+.. _screencasts:
+
+***********
+Screencasts
+***********
+
+This section provides links to screencasts that explain how to install gprMax, demonstrate some of its key features, and give example models showing applications.
+
+Installation
+------------
+
+* How to install gprMax for end users on Microsoft Windows (https://youtu.be/bj3cRwH107U)