sphinx/conf.py: Fix warning with latest pydata-sphinx-theme release

Latest pydata-sphinx-theme release prints a new warning about
an accessibility issue and recommends to set to False the
navigation_with_keys theme option.

It fixes CI jobs on Jenkins as sphinx warnings are treated as
errors.

See https://github.com/pydata/pydata-sphinx-theme/issues/1492

swh/docs/sphinx/conf.py

@@ -142,6 +142,7 @@ html_theme_options = {
         },
     ],
     "navbar_persistent": ["search-button"],
+    "navigation_with_keys": False,
 }
 
 html_logo = "_static/software-heritage-logo-title.svg"