Recommend separate apps folder for dev

[PVince81]

    > In a more general note, I wonder if we should advise people to always configure a second apps folder, it would make manual upgrades more smooth.

All the docker devs setups we use are using this.
But the documentation should be updated
https://docs.nextcloud.com/server/latest/developer_manual/getting_started/devenv.html#check-out-external-shipped-apps

Even for regular setups, I think we should push that forward imho
https://docs.nextcloud.com/server/latest/admin_manual/apps_management.html#using-custom-app-directories

Originally posted by @skjnldsv in nextcloud/server#35622 (comment)

[PVince81]

There's also the potential for a bigger impact if we make this a general guideline also for installation setups, because it would make manual upgrades easier by only having to care about the extra apps folder where app store apps get installed and third party ones. Currently the docs tell the people to carefully find out which apps are bundled and which ones are not.

This doesn't have to be done as part of this ticket though.

[PVince81]

closing in favor of nextcloud/standards#9

assuming that this will also make it appear somehow in the dev docs