doc: add version switcher flyout

Backport the bottom-left version switcher from unstable. Overrides the empty
versions.html shipped by sphinx_rtd_theme 3.x with _templates/versions.html and
populates html_context['versions']; styled by the theme CSS, toggled by theme.js.

Author: the-hampel

doc/_templates/versions.html

@@ -0,0 +1,25 @@
+{# Version switcher flyout.
+
+   sphinx_rtd_theme 3.x ships an empty versions.html (the flyout is injected by
+   the ReadTheDocs addons when hosted there, and is absent on self-hosted
+   builds). layout.html still does `{% include "versions.html" %}`, so this
+   override revives the classic bottom-left "v: <version> ▾" widget. It is
+   styled by the theme's existing .rst-versions CSS and toggled by theme.js;
+   the entries come from html_context['versions'] in conf.py. #}
+{%- if versions %}
+<div class="rst-versions" data-toggle="rst-versions" role="note" aria-label="{{ _('Versions') }}">
+  <span class="rst-current-version" data-toggle="rst-current-version">
+    <span class="fa fa-book"> {{ _('Versions') }}</span>
+    v: {{ current_version }}
+    <span class="fa fa-caret-down"></span>
+  </span>
+  <div class="rst-other-versions">
+    <dl>
+      <dt>{{ _('Versions') }}</dt>
+      {%- for slug, url in versions %}
+      <dd><a href="{{ url }}">{{ slug }}</a></dd>
+      {%- endfor %}
+    </dl>
+  </div>
+</div>
+{%- endif %}

doc/conf.py

@@ -88,7 +88,22 @@
 
 html_show_sphinx = True
 nbsphinx_allow_errors = True
-html_context = {'header_title': '@PROJECT_NAME@'}
+html_context = {
+    'header_title': '@PROJECT_NAME@',
+    # Version switcher flyout (rendered by _templates/versions.html, which
+    # overrides the empty versions.html shipped by sphinx_rtd_theme 3.x). The
+    # list is static -- update it when a new release line is published.
+    # display_lower_left enables the flyout on older theme versions that gate
+    # the include on it; harmless on 3.x.
+    'display_lower_left': True,
+    'current_version': version,
+    'versions': [
+        ['latest',   'https://triqs.github.io/solid_dmft/latest/'],
+        ['unstable', 'https://triqs.github.io/solid_dmft/unstable/'],
+        ['4.0.x',    'https://triqs.github.io/solid_dmft/4.0.x/'],
+        ['3.3.x',    'https://triqs.github.io/solid_dmft/3.3.x/'],
+    ],
+}
 html_static_path = ['@CMAKE_CURRENT_SOURCE_DIR@/_static']
 html_sidebars = {'index': ['sideb.html', 'searchbox.html']}