Add FAQ Section or Notice in FTUE/Add Calendar page about what we support

[malini]

Is your feature request related to a problem? Please describe.
We get questions often about users asking to connect local calendars or calendars we do not yet support. It is not clear that we do not support them; it is simply implied that we only support CalDAV and Google Calendar, and most people don't know what CalDAV is.

It would be good to explain:

• we do not support local calendars like those set up directly in Thunderbird
• we do not yet have Apple Calendar support, but they can follow along in Apple Calendar support #506
• we do not yet have Microsoft Exchange support, but they can follow along in Add Microsoft support #635

Describe the solution you'd like
Perhaps we should have an FAQ section detailing this information, or have it listed in the "Calendar settings" page
Describe alternatives you've considered