Tidy up of runsphinx template. Refs #9639
Allow single file selection for sphinx run

Refs #9639
Enable sphinx to be run without MantidPlot.

Refs #9639
Separate the Sphinx doctree directories by bulder type.

Each builder type now has a separate environment so that the order of
running the targets doesn't make a difference. Shared doctree directories
meant that running doctest first just in vanilla python wouldn't
generate the screenshots and running the html target wouldn't reread
the unchanged source files to generate the images. It forced doctest
to run in MantidPlot and do the screenshot generation.

Refs #9639
Run docs-test target with vanilla python.

Refs #9639
Add individual rst files as ctest targets.

They can be disabled by turning off the SEPARATE_DOC_TESTS cmake option.
Refs #9639
Turn off the separate documentation tests on the build servers

They will instead just run the single global target through
MantidPlot that will be much faster than running them indivually.
Refs #9639
Update ctest doc tests for multi-config generators.

Refs #9639
Use a separate doctree directory for each file in single file mode.

Refs #9639
Merge branch 'feature/9639_doc_tests_inside_ctest' into develop