Default app setting on a user basis

[JammingBen]

WHAT Needs to be Documented?

https://github.com/owncloud/core/pull/39600 introduces the possibility for users to have their own default app (basically overwriting the config defaultapp).

WHERE Does This Need To Be Documented (Link)?

I'm actually not sure. As discussed and stated in https://github.com/owncloud/core/pull/39600#issuecomment-995617737, this setting can be adjusted by admins via occ command, but is not intended to be used this way. It's more of a way for users to set it themselves. Currently we plan this for having in the web app only.

(Optional) What Type Of Content Change Is This?

• [x] New Content Addition
• [ ] Old Content Deprecation
• [ ] Existing Content Simplification
• [ ] Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• [ ] Admin Manual
• [ ] Developer Manual
• [x] User Manual
• [ ] Android
• [ ] iOS
• [ ] Branded Clients
• [ ] Desktop Client
• [ ] Other