conf.py: add a link in the sidebar to upstream documentation page

conf.py

@@ -125,6 +125,13 @@ todo_include_todos = False
 #
 html_theme = 'alabaster'
 
+html_sidebars = {'**': ['navigation.html', 'searchbox.html']}
+html_theme_options = {
+    'extra_nav_links': {
+        "← Guide de l'administrateur technique":
+        "https://doc-publik.entrouvert.com/tech/"}
+}
+
 # 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.