NextBox: Missing documentation

To collect some recent needs:

• add more details for the backup/restore steps (and contents)
• Factory Reset: be more clear about what actually is deleted
• How to migrate from another Nextcloud instance (some guidelines and best practices)
• add description on how to DIY a nextbox including flashing the image
• add "pseudo" mirroring solution description via federation + shared folders see https://support.nitrokey.com/t/nextbox-minimal-mirroring-solution/3196?u=daringer
• document workaround on how to use an own tls-ceritficate
• improve port-forwarding docs to only forward to a specific IP (the nextbox)
• document in more detail how the TLS certificate is acquired (https://github.com/Nitrokey/nextbox/issues/23)
• tech-details: describe the software-architecture and update procedure in more detail for the nextbox and it's components (https://github.com/Nitrokey/nextbox/issues/22)
• tech-details: document further nextbox hardening options: https://github.com/Nitrokey/nextbox/issues/25
• tech-details: how secure is the nextbox: https://support.nitrokey.com/t/security-on-the-nextbox/3586
• faq: add the information that IPv6 might not work from certain networks (e.g., some obile networks in germany)
• faq: remote-access add reminder for possible private IPv4 address and CG-NAT Drawbacks
• faq: which system users are created for the nextbox off-shelf + default passwords (there are none),
• FAQ: on Fritz-Box: nextbox via LAN, client via WiFi nextbox.local does not work ... alternatives nextbox.fritz.box + ip
• FAQ: WiFi
• FAQ: htaccess error = not enough power USB
• FAQ: software is a 64bit OS
• FAQ: how secure is the NextBox (unattended-upgrades frequency, and linking https://www.debian.org/security/ + https://www.debian.org/security/faq)
• https://support.nitrokey.com/t/nextbox-guided-dynamic-dns-cant-enable-https/3194/2?u=daringer
• document https://support.nitrokey.com/t/nextbox-guided-dynamic-dns-cant-enable-https/3194/7
• add faq for Android and nextbox.local / mDNS in general
• add faq for getting help on apps & purely nextcloud related stuff
• document how to run occ
• add proper wording for "button", "hole" and how-to-press-the-button
• add FAQ for "MyFritzBox" service taking port 443 ... https://support.nitrokey.com/t/kein-zugriff-mehr-auf-nextbox/3256/4
• extend FAQ and other hints for "power supply external hard-drive" with remark that a powered-usb-hub translates also to exactly this
• extend "getting-started" with hint for "NextBox-App" inside Nextcloud
• add FAQ for "mirrored NextBox not supported"
• FAQ: how to use the button properly (also add to factory reset)
• add description how to replace the hard-drive (which files to copy, how to partition, etc)
• FAQ: how to update my OS -> describe update mechanisms and that upgrades are unattended
• FAQ: MyFritz! and "Fritz Services"
• add FAQ for:
• add https://www.raspberrypi.org/documentation/hardware/raspberrypi/usb/README.md somewhere USB related
• FAQ: missing imagemagick in php
• add "walkthrough" to Remote Access (https = needs domain, deciding help for quickstart vs. dns)