Add search path for CMake to find Sphinx on Windows.

Refs #9559
Simplify Sphinx runner script.

It just calls the sphinx.main() function directly without the need to
go through pkg_resources.
Refs #9559
Group the documentation targets in Visual Studio.

Refs #9559
Use .summary() instead of deprecated .getWikiSummary()

Refs #9559
Ensure MantidPlot exits with 0 status code on script success.

Refs #9559
Merge remote-tracking branch 'origin/feature/9559_fix_sphinx_on_windows'