Adds callout to introduce Markdown

[pgmccann]

Borrows heavily from https://github.com/swcarpentry/python-novice-gapminder/blob/2aef01ba47cbf88e9c607f920b83b7dc59d7044e/episodes/01-run-quit.md. Requires update to varnish so that table within callout is styled properly.

I'm not fully across the new lesson platform, so I'm not entirely clear how to update varnish for local testing with my fork of this lesson.

[github-actions]

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🔍 Inspect the changes: https://github.com/swcarpentry/git-novice/compare/md-outputs..md-outputs-PR-1018

The following changes were observed in the rendered markdown documents:

 04-changes.md | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 md5sum.txt    |  2 +-
 2 files changed, 51 insertions(+), 1 deletion(-)

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⏱️ Updated at 2024-10-02 13:42:00 +0000

[martinosorb]

Hi @pgmccann, I have a few concerns: it's a large table that may be a bit bulky here. I am not sure if the "callout" actually works due to Varnish, as you say (I remember working on this on the python-novice-gapminder side). Lastly, there is no introduction to it: it should be put in context and explained why we are showing this table through an introduction to what markdown is.