docs: Maintenance Menu section

The introduction of the Maintenance Menu as a seucrity-senstivie
features needs proper documentation.

This commit:
* replaces the "Factory reset" section in Tinkering with the new
  "Maintenance Menu" section

Depends on:
BitBoxSwiss#317

docs/tinkering/maintenance-menu.md

@@ -0,0 +1,56 @@
+---
+layout: default
+title: Maintenance menu
+parent: Tinkering
+nav_order: 400
+---
+## Maintenance menu
+
+Using the BitBoxApp as user interface requires the Middleware on the BitBoxBase to always be available.
+Only then the node management dashboard is reachable the user can log in with the management password to be able to interact with the device.
+
+In cases where this is not possible, e.g. because of a forgotten password or an internal error that prevents the Middleware to be reachable, the Maintenance Menu is available locally on a HDMI screen as a fallback.
+
+![Maintenance Menu on HDMI port](maintenance-menu.png)
+
+### Features
+
+The Maintenance Menu serves both the initial factory setup and a user-triggered factory reset.
+
+#### Factory setup
+
+The goal of the factory setup is to allow basic tasks like loading pre-synched Bitcoin blocks without knowing the device credentials.
+
+Current features:
+
+* Creating a Bitcoin / Electrs blocks snapshot to an external USB drive
+* Loading snapshot from an external USB drive to the internal SSD
+* Delete any files created during device testing (SSH / SSL certificats, application data, logs etc.)
+* Wiping the factory credentials
+
+#### Factory reset
+
+The following options allow the user to reset the device in case of a forgotten management password or an internal configuration issue.
+
+* **Authentication reset**: the BitBoxApp will allow to set a new management password on next connect
+* **Configuration reset**: factory defaults are restored (Redis database, new SSH and SSL keys...), without touching any Bitcoin or Lightning data
+* **Disk image reset**: in case an over-the-air update is no longer possible, an official, signed  BitBoxBase release image named `update.base` can be flashed from the USB flashdrive
+
+It is also possible to flash a custom-built, unsigned firmware update using the "Disk image reset" functionality, but only if the BitBoxBase has been previously configured to accept unsigned updates (calling `bbb-config.sh enable unsigned_updates` from the command line).
+This setting cannot be set from the Maintenance Menu.
+
+### Authentication
+
+Access to the Maintenance Menu is a sensitive operation and requires the possession of a "maintenance token".
+This token is stored in `.maintenance-token` on the USB flashdrive, and updated with every system configuration backup.
+It is a random number and acts as the authentication secret.
+The BitBoxBase itself stores the SHA256 hashes of all previously created token in `/data/maintenance-token-hashes`.
+Deleting this file revokes all previously issued maintenance token.
+
+On every boot, the systemd [`startup-maintenance.service`](https://github.com/digitalbitbox/bitbox-base/blob/master/armbian/base/scripts/startup-maintenance.service) calls the script [`systemd-startup-maintenance.sh`](https://github.com/digitalbitbox/bitbox-base/blob/master/armbian/base/scripts/systemd-startup-maintenance.sh) which checks if a USB flashdrive containing a maintenance token is plugged into the BitBoxBase.
+The token is validated by comparing its SHA256 hash against the local list of previoulsy created token hashes.
+If a valid token is present, the Maintenance Menu is displayed on tty2 over HDMI.
+Otherwise, the script exits immediately, without the possibility of any user interaction.
+
+The inital BitBoxBase image flashed on new devices contains a "Shift factory token" that allows access to the menu with a maintenance token kept confidential by BitBoxBase personnel.
+This factory token is [clearly marked as such](https://github.com/digitalbitbox/bitbox-base/blob/master/armbian/base/config/signatures/maintenance-token-hashes) and is removed either manually when running "Finish factory setup..." or when the first system configuration backup is created, usually at the end of the Setup Wizard.