ENH: Remove HTML and Liquid comments from index.md

Remove HTML and Liquid comments from `index.md`.

The Liquid comment is visible in the new Workbench infrastructure index
page. None of these comments are necessary, and were present since the
very first commit (8f66f41), as they were present in the default
contents of the `index.md` file in the legacy Carpentries infrastructure
files.

index.md

@@ -37,10 +37,6 @@ _Note: All of this may sound complicated, but we'll explain things step-by-step
 ![course_flow](fig/index/Course_flow_0.png)
 
 
-<!-- this is an html comment -->
-
-{% comment %} This is a comment in Liquid {% endcomment %}
-
 > ## Prerequisites
 > Attendees must have some base familiarity with Python and NIfTI images in order to comfortably progress through the lesson
 {: .prereq}