Send event invitation to the person who booked an appointment

[Bubu]

Is your feature request related to a problem? Please describe.
After clicking the confirmation link for an appointment, I'd expect to be mailed a proper event invitation to add to my local calendar.

Currently this doesn't happen.

Describe the solution you'd like
See above.

Describe alternatives you've considered
Addding the event to the confirmation email as described here: #3852 (comment)

This would probably have way more potential for confusion if users forget to also click the confirmation link.

Additional context
See #3852

[42ske]

In addition the confirmation mail should contain:

• date and time of the appointment as text
• free text which can be set in the appointement's options
• a link to cancel the appointment again
• an ics-file with the appointment including infos which can be defined in the options

I described my usage scenario over here: https://help.nextcloud.com/t/feature-requests-for-appointments-in-calendar/134767