Explain the Purpose of the well-known services for Nextcloud

[KilianHanich]

At the state of now Nextcloud supports quite a few Auto Discovery Services like WebDav, Webfinger etc.

It would be good for Administrators to explain what Nextcloud uses these Services for.

While the purposes of some services for Nextcloud are pretty self-explanatory (like WebDav), that is not the case for others (like Webfinger).

[ChristophWurst]

@ArtificialOwl for webfinger / nextcloud/server#11396

[KilianHanich]

So, from what I got from this Pull Request, is that it (EDIT: webfinger) is only used when an app registers to do something with it.

Good to know, but I think this should be documented in the Admin manual. As an admin of a Nextcloud instance I kinda want to know what kind of APIs Nextcloud uses and supports to be able to properly judge the situation.

[joshtrichards]

Related: nextcloud/server#24702

[joshtrichards]

Related: #6885