docs: Move homepage link to TOC tree.

This lets us avoid maintaining a forked copy of sphinx_rtd_theme’s
sidebartitle block.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

docs/_templates/layout.html

@@ -1,29 +1,5 @@
 {% extends "!layout.html" %}
 
-{% block sidebartitle %}
-    {%- if logo and theme_logo_only %}
-        <a href="{{ pathto(root_doc) }}">
-            <img src="{{ pathto('_static/' + logo, 1) }}" class="logo" alt="{{ _('Logo') }}"/>
-        </a>
-    {%- endif %}
-
-    {%- if theme_display_version %}
-        {%- set nav_version = version %}
-        {%- if READTHEDOCS and current_version %}
-            {%- set nav_version = current_version %}
-        {%- endif %}
-        {%- if nav_version %}
-            <div class="version">
-                {{ nav_version }}
-            </div>
-        {%- endif %}
-    {%- endif %}
-
-    <a class="reference external home-link" href="https://zulip.com/">Zulip homepage</a>
-
-    {%- include "searchbox.html" %}
-{% endblock %}
-
 {% block document %}
     {#
     # This allows us to insert a warning that appears only on the development