docs: update doc screenshots with new sett settings tab

To be done after #233 (closed) and #237 (closed)

Message received from Sabine and Martin

latest SETT version 2.6 has a different front screen with a settings tab. Also a nag screen on exit by default.

The third graphic shows the first screen on starting SETT with the fourth tab. So any doc which includes the start screen (or other top level screen) will need updating.

I think the nag screen appears if you try to exit without ever having changed and saved the configuration and so the user's config.json file does not exist. Once it exists you do not see the nag screen on exit if you have changed nothing.

The problems to fix:

• update doc screenshots with new sett settings tab.
• The "nagging tab" is because they changed some settings and did not save