Resolve documentation build errors

Eliminated all the warnings and errors in the documentations.

Signed-off-by: Gabor Toth <gabor.toth2@arm.com>
Change-Id: I355b0db832e62d7eb4f0968c309e547ae7a51dc3
diff --git a/docs/conf.py b/docs/conf.py
index feeca38..3bdd93e 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -91,7 +91,7 @@
 
 #Add custom css for HTML. Used to allow full page width rendering
 def setup(app):
-    app.add_stylesheet('css/custom.css')
+    app.add_css_file('css/custom.css')
 
 
 # -- Options for autosectionlabel --------------------------------------------