Build documentation & tests on RHEL6

Refs #9593
Keep python 2.6 compatibility in sphinx directives.

The positional argument index is required in format strings in python 2.6
Refs #9593
On a clean build tar up the documentation.

This will make it easier to ship around and deploy.
Refs #9593
Fix the dependencies on the docs-* targets.

Refs #9593
Add mantidqtpython as MantidPlot dependency

Refs #9593
Add RemoteAlgorithms as a framework library.

Refs #9593
Use screenshots in doctest build too.

If we dont and you run the doctest before html generation then you don't
see a screenshot because sphinx won't have reprocessed the file.
Refs #9593
Merge remote-tracking branch 'origin/feature/9593_docs_build_in_jenkins_scripts'