readthedocs: Add proper backlink to the Zulip homepage.

Now that we are starting to link this pages from the landing page's
top navigation, it makes sense to have proper backlinks to the
homepage so that there is some continuity when the user clicks on
a link that takes them to a ReadTheDocs page from the main website.

docs/_static/theme_overrides.css

@@ -9,3 +9,7 @@
 .wy-table-responsive {
     overflow: visible !important;
 }
+
+.wy-side-nav-search > a.home-link {
+    font-weight: 100;
+}

docs/_templates/layout.html

@@ -1,4 +1,29 @@
 {% extends "!layout.html" %}
+
+{% block sidebartitle %}
+    {%- if logo and theme_logo_only %}
+        <a href="{{ pathto(master_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

tools/check-templates

@@ -26,6 +26,8 @@ EXCLUDED_FILES = [
     "templates/zerver/emails/missed_message.source.html",
     # Previously unchecked and our parser doesn't like its indentation
     "static/assets/icons/template.hbs",
+    # The parser does not like the indentation of custom ReadTheDocs templates
+    "docs/_templates/layout.html",
 ]