[DOC] Correct requirements for doc chain building

- Remove unecessary requirements
- It leverages the latest lfdocs-conf (0.7.7) updates
- It now loads all centralized upper contraints from ONAP doc
- Correct conf.py to use app.add_css_file()

Issue-ID: DOC-765

Signed-off-by: Eric Debeau <eric.debeau@orange.com>
Change-Id: Ie8aa095bc9ac263a754734ea7d9bfff427ddcca0
diff --git a/docs/conf.py b/docs/conf.py
index 5371015..d211470 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -12,4 +12,4 @@
 html_last_updated_fmt = '%d-%b-%y %H:%M'
 
 def setup(app):
-    app.add_stylesheet("css/ribbon.css")
+    app.add_css_file("css/ribbon.css")