doc: tell sphinx we don't have any static files

We don't have any static files to copy over, so comment out the config
option about it. Silences warning:

WARNING: html_static_path entry '/home/jani/src/maintainer-tools/_static' does not exist

Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

conf.py

@@ -153,7 +153,7 @@ html_theme = 'sphinx_rtd_theme'
 # 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".
-html_static_path = ['_static']
+# html_static_path = ['_static']
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied