Fixes for docs build:
* Use doc/requirements to follow changes in OpenStack CI for the docs
building, add a tox environment for this.
* Remove reference to "project's download page", the link was removed in
change I2db594d279e1229e5b1600cecad86fe0c3612115 and now we have a
dead link that errors building if warnings are treated as errors.
* Change I2db594d279e1229e5b1600cecad86fe0c3612115 added a column for
DB2 header but not for any entries, enhance table with extra column.
This fixes building of the table if warnings are treated as errors.
* Update to sphinx 1.6.x since that is what is used in OpenStack CI job,
remove issuetracker, it does not work with sphinx 1.6 anymore.
* Disable html_static_path - the content does not exist
* Use python3 for docs building