Moves app management API chapter from User Management to Apps management section

[joshtrichards]

I'm not sure why this chapter is in the user management section of the Admin Manual. I think it got added a million years ago alongside user management API. Probably should get moved to the Apps Management section.

https://docs.nextcloud.com/server/latest/admin_manual/configuration_user/instruction_set_for_apps.html

Will likely require:

• tidying up of the overview which mentions it: https://docs.nextcloud.com/server/latest/admin_manual/configuration_user/user_provisioning_api.html
• moving existing https://docs.nextcloud.com/server/latest/admin_manual/configuration_user/instruction_set_for_apps.html to be underneath Apps management section
• Changing title form "Instruction set for apps" to something like "Apps management API"