The project-specific docs are published independently so we need
to maintain the links into those subtrees (/stx-*), also
for the api-ref (/api-ref/stx-*) and releasenotes (/releasenotes/stx-*).
Also add docs env to the default list in tox.ini.
Change-Id: If68e6c984d684af05124b3252a8ff3c4b44b6d82
Signed-off-by: Dean Troyer <dtroyer@gmail.com>