doc: Use local Open Sans fonts instead of querying Google

The salt-doc package creates a potential privacy breach by fetching the
Open Sans font from the external website fonts.googleapis.com at
runtime.

Drop fetching the font from fonts.googleapis.com and let salt-doc depend
on fonts-open-sans to provide the font.

Signed-off-by: Benjamin Drung <benjamin.drung@profitbricks.com>