btrfs-progs: docs/conf.py: enable navigation_with_keys on RTD

Feature from https://github.com/sphinx-doc/sphinx/pull/2064 enable
navigation to be able to navigate documentation using the arrow keys.

Pull-request: #754
Author: Martin <spleefer90@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/conf.py

@@ -37,6 +37,10 @@ exclude_patterns = ['_build']
 # a list of builtin themes.
 html_theme = 'sphinx_rtd_theme'
 
+html_theme_options = {
+    'navigation_with_keys': True
+}
+
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".