Add horizontal scrollbar to TOC when required

[geographika]

Currently due to long constant names there are overlaps between the TOC and the content at https://mapserver.org/mapscript/mapscript-api/index.html

This pull request ensures a horizontal scrollbar is used (when required) to avoid this overlap.

I was hoping the local TOC could be modified on that page, but it does not appeat to be possible. Failed attempts included adding:

.. toctree::
   :hidden:
   :maxdepth: 2

.. contents:: Table of Contents
   :local:
   :depth: 2

And also trying to modify localtoc2.html similar to how the Read the Docs theme works with the global TOC, but this does not work with toc:

{%- if display_toc %}
  <h3>{{ _('Current Table Of Contents') }}</h3>
  {{ toc(maxdepth=2) }}
{%- endif %}

[jmckenna]

thanks @geographika