Workflow documentation is now in infra-manual

Replace URLs for workflow documentation to appropriate parts of the
OpenStack Project Infrastructure Manual.

Dropped sections that duplicate each other after the change.

Co-Authored-By: Dmitry Tantsur <dtantsur@redhat.com>
Change-Id: I233df03368dbb4b7805d6dce28de00fa037353c7
Merge "Workflow documentation is now in infra-manual"