Fix Table of Contents (2) not showing top part of table

[coatless]

This adds a top margin of 10em to ensure the table of contents initial section is visible and not cutoff/inaccessible.

Before:

After:

This can quickly be explored by adding a Python cell with:

from IPython.display import display, HTML
display(HTML("""
<style>
#toc-wrapper { 
z-index: 90; 
position: fixed !important;
display: flex;
flex-direction: column;
overflow: hidden;
padding: 10px;
border-style: solid;
border-width: thin;
background-color: #fff;
/* default - alterable via nbextension-configurator */
top: 10em;}
</style>"""))

Close #1651