[docs] Transition to local docs build job.

[docs] Transition to local docs build job.

Docs builds have transitioned to using local
build jobs. This allows Yardstick docs to be
built and verified locally.
The time taken to build and verify docs in the
gate for new changes should be reduced, as the
docs for each project will be built separately.

This change also allow contributers to test
their docs builds locally, using tox::

    tox -e docs
    tox -e docs-linkcheck

TODO: Update the dev guide to include new tox
environment

Related: DOCS-191
JIRA: YARDSTICK-1415
Signed-off-by: Emma Foley <emma.l.foley@intel.com>
Change-Id: Ib8bab85475dcd5127a2c41a2d55dce8f8d9da574