docs: remove broken Channel9 video iframe (fixes #1454)

The Channel9 video link has been broken since Microsoft retired
the Channel9 platform. Remove the broken iframe and introductory
text since we do not have a replacement video link.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

docs/index.md

@@ -67,10 +67,6 @@ use multiple features together:
  * [Anonymizing JSON](tutorials/JsonAnonymizer.html) - implements a function to anonymize a `JsonValue` from FSharp.Data.
    The tutorial demonstrates pattern matching on `JsonValue`.
 
-Below is a brief practical demonstration of using FSharp.Data:
-
-<div style="padding-left:20px"><iframe src="https://channel9.msdn.com/posts/Understanding-the-World-with-F/player" width="640" height="360" allowFullScreen frameBorder="0"></iframe></div>
-
 ## Reference Documentation
 
 There's also [reference documentation](reference) available. Please note that everything under