Mention the .ics alternative for the Google calendar

First of all, many thanks for vdirsyncer.

I tried first with a Radicale CalDAV server, and everthing worked fine.

Then I tried to backup my Google Calendar. I didn't give it a 2nd thought and tried to modify the vdirsyncer configuration. I went to the Google console, enabled the API, added myself as a test user, etc. In the end, I couldn't manage, so I gave up. I am guessing that Google does not really want you to use any app with their calendar.

But then, afterwards, I realised that you can get a URL like this for your Google calendar:

https://calendar.google.com/calendar/ical/user%40gmail.com/private-0123456789abcdef01234567890abcde/basic.ics

It only provides read access, but that is fine, as I only wanted to backup my calendar in the first place.

In order to avoid frustration and loss of time for the next user, I would mention in https://vdirsyncer.pimutils.org/en/stable/config.html#google that, if you only want to backup your calendar, you could just use the easier HTTP method instead.

Open the Google Calendar section at config.html#google and review how authentication and backup use cases are described. Add a note that read-only calendar backups can use the Google Calendar .ics URL through the easier HTTP method; the documentation is done when this alternative is clear to readers who only need backup access.