AppConfig is not recommended everywhere

[SMillerDev]

Since nextcloud 29 there is: AppConfig
But if you're looking at the configuration docs there is no mention of this, and implementing those recommendations will set you up to use deprecated methods that are going away.

The documentation should probably recommend AppConfig over IConfig.

[ChristophWurst]

Indeed the new APIs were documented with #11617 but existing examples need adjustment.

[ChristophWurst]

This needs backports for 30 and 29

[Duo123418]

Hi, I would like to work on this issue by updating the documentation to reflect the recommended usage of AppConfig in place of IConfig. Could I proceed with this?

[SMillerDev]

As far as I know anyone can make a pull request here, so I'd say you can proceed.