Update the gCal integration documentation

[Valentina R]

The documentation is outdated and many new customers keep having issues. I point them to this thread with screenshots, but even that might be a bit outdated - but at least it works.

And don’t forget to put a warning that the calendar ID must be selected - because if left as N/A, nothing works as it should, especially staff calendars.

[Valentina R]

Add this to the documentation.

[Valentina R]

Here is the updated version as of 31.05.2023 with new screenshots:

https://docs.google.com/document/d/1RDYo2b80KibsG2Lq-FWHK6UWRQ0ZQgcAC2rjQr9Ir3Q/edit?usp=sharing