How to add Sphinx-generated index to the sidebar when using Read the Docs theme?

I'd like to have a link to the automatically generated index in the sidebar when using sphinx-rtd-theme. I've tried adding it to the toctree:

.. toctree::

    first
    second
    Index <:ref:`genindex`>

but this resulted in

WARNING: toctree contains reference to nonexisting document u':ref:`geinindex`'

from Sphinx and no other effect.

I think I could simply hardcode the index in the theme layout.html file, but perhaps there is some better way, not involving modifying the standard theme?

TIA for any hints!

It's easy if you understand how Sphinx and Jinja work. Unfortunately the Sphinx docs on templating don't give you enough info if you don't. In short, you'll have to override the template:

• Make sure you have a _templates folder under your sphinx docs folder.
• Make sure it is listed in your conf.py, e.g. templates_path = ['_templates']
• Create a file inside the folder called layout.html.

• Put this snippet inside and save. The exclamation point/mark forces jinja to use the parent template. Don't forget it, or you'll get a recursion error. You only need to override the menu block.

  {% extends "!layout.html" %}
  
    {% block menu %}
      {{ super() }}
      <a href="genindex.html">Index</a>
    {% endblock %}