Add branching-model.html to website/index.html
Add branching-model.html to website/index.html

The branching-model.html should be added to the web index.html

Closes-Bug: #1644958
Change-Id: I810a2816b6e331d6b29a42c60a9589a294308d6b

openstack/training-guides,openstack/training-guides