Add screenshot of Graph View to tutorial

[wxtim]

After cylc/cylc-ui#74 is closed Add an screencap to the tutorial here: https://github.com/cylc/cylc-doc/blob/master/src/tutorial/runtime/introduction.rst#the-cylc-gui

[oliver-sanders]

This is now easy to do.

1. Setup an e2e or component test in cylc-ui which generates the required screenshots.
2. Setup a GitHub action to synchronise screenshots from cylc-ui to cylc-doc when they change.

Note, the task/job icon screenshots used in the docs are generated by component tests in cylc-ui, but are not currently automatically synchronised with cylc-doc.