This commit adds PDF documentation build target 'pdf-docs' that will
build PDF versions of our docs. And this commit also updates 'docs'
target to use sphinx-build command as well as 'pdf-docs' target.
This is one of the Train community goals:
https://governance.openstack.org/tc/goals/selected/train/pdf-doc-generation.html
Change-Id: Iae6bc9ae92fe61385c424923eaad35dc449880e9
Story: #2006070
Task: #35455
This commit updates the docs theme to the new one. The new theme is the
standard of the openstack document. There is no reason to avoid to use
it.
Change-Id: If17ce35bd2ba50c3e1ae6e1567dcbe4be59d5189