Configure RTD theme to use titles only

0e55968d removed :titlesonly: from the
root toctree, which caused sphinx_rtd_theme to switch the mode it uses
to display *all* sidebars.

This config change reverts this behavior, but keeps the content of
pages unchanged.

swh/docs/sphinx/conf.py

@@ -112,6 +112,7 @@ html_favicon = "_static/favicon.ico"
 html_theme_options = {
     "collapse_navigation": True,
     "sticky_navigation": True,
+    "titles_only": True,
 }
 
 html_logo = "_static/software-heritage-logo-title-motto-vertical-white.png"