Add sphinxext-opengraph to requirements

Adds Open Graph meta tags so LinkedIn/Facebook link previews show a proper title, image, and description instead of the broken preview.

Author: sorochak

requirements.txt

@@ -109,6 +109,7 @@ sphinxcontrib-htmlhelp==2.0.0
 sphinxcontrib-jsmath==1.0.1
 sphinxcontrib-qthelp==1.0.3
 sphinxcontrib-serializinghtml==1.1.5
+sphinxext-opengraph==0.9.1
 SQLAlchemy==2.0.15
 stack-data==0.3.0
 tabulate==0.8.10