doc: bring back prev/next buttons
Some ACRN docs (in particular the HLD) has content nested more than the
4 levels shown in the sidebar navigation.  So when looking at those
documents, it's hard to get to next chapter.  Bring back the
previous/next links to fix this situation.
@@ -166,7 +166,7 @@ else:
'analytics_id': '',
'logo_only': False,
'display_version': True,
- 'prev_next_buttons_location': 'None',
+ #'prev_next_buttons_location': 'None',
# Toc options
'collapse_navigation': False,
'sticky_navigation': True,
