Add info/tips/warning boxes

[dpshelio]

Similarly how we can do it in gh markdown:

Important

hello there

but that it works on the jupyter (and the rendered notes).

I've seen two options:

• Markdown tables with emojis

  ❗ Important
  This is an important message

• Or adding html block (with the warning that this is not rendered on gh issues)

  <div class="alert alert-block alert-warning">
  **NOTE:**
  Find the docs [here](http://example.com/).
  </div>

A third option could be to add some css magic on our custom css for the course.