[stable25] Add documentation for iMIP API

[miaulalala]

Fixes #9247

[ChristophWurst]

Fixes #9247

I'd say there are two aspects of it

1. Apps that provide a calendar
2. Apps that use the provided calendars

Right now it covers 1), although it's a must, not may in the current API. Every writable calendar has to handle iMip.

[ChristophWurst]

• Todo: also note the new method at https://docs.nextcloud.com/server/latest/developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_25.html#back-end-changes should there be an app that already implemented ICreateFromString (breaking change)

[miaulalala]

• Todo: also note the new method at https://docs.nextcloud.com/server/latest/developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_25.html#back-end-changes should there be an app that already implemented ICreateFromString (breaking change)

Added to the collective ticket at nextcloud/server#32117

[miaulalala]

this needs to go into 25 only.