toctree nested drop down

I have a toctree in my index.rst that looks something like this:

.. toctree::
   :maxdepth: 2

   cat
   dog
   moose

I am looking to to nest the contents of my toctree similar to how it is done here with 'api documentation':

So ultimately make something like:

.. toctree::
   :maxdepth: 2
   :dropdown Animals
     cat
     dog
     moose

But I cannot seem to find anything that does this in the docs.

This behaviour of the toctree in the sidebar is is a feature of the Read the Docs theme (https://github.com/snide/sphinx_rtd_theme)

Install it with pip install sphinx-rtd-theme

The theme has the option collapse_navigation that controls whether to auto-collapse the tree when navigating to another part of the documentation or not.

# -- Options for HTML output ----------------------------------------------

# The theme to use for HTML and HTML Help pages.  See the documentation for
# a list of builtin themes.
html_theme = 'sphinx_rtd_theme'

# Theme options are theme-specific and customize the look and feel of a theme
# further.  For a list of options available for each theme, see the
# documentation.
html_theme_options = {
    "collapse_navigation" : False
}

indexr.rst:

#################
  Title
#################

Animals
=======

.. toctree::
   :maxdepth: 2

    animals/index

Flowers
=======

.. toctree::
   :maxdepth: 2

   flowers/index

animals/index.rst:

####################
  Animals
####################

.. toctree::
   :maxdepth: 2

   cat
   dog
   moose