[DOCS] Document Maintenance Mode in a new System Settings section (#5169)

* [DOCS] Document Maintenance Mode in a new System Settings section

I am planning to move more administration related system setting information here from Configuration and Api, where they don't really belong

Releases: main, 13.4, 12.4

* Update Documentation/Administration/SystemSettings/MaintenanceMode/Index.rst

* Apply suggestions from code review

Co-authored-by: Stefan Frömken <froemken@gmail.com>

---------

Co-authored-by: Stefan Frömken <froemken@gmail.com>

Author: linawolf

Documentation/Administration/Index.rst

@@ -35,6 +35,7 @@ TYPO3 administration
 
     Installation/Index
     Upgrade/Index
+    SystemSettings/Index
     UserManagement/Index
     Troubleshooting/Index
     */Index

Documentation/Administration/SystemSettings/Index.rst

@@ -0,0 +1,13 @@
+:navigation-title: System Settings
+..  include:: /Includes.rst.txt
+..  _system-settings:
+
+========================================
+TYPO3 system settings for administrators
+========================================
+
+..  toctree::
+    :glob:
+    :titlesonly:
+
+    */Index

Documentation/Administration/SystemSettings/MaintenanceMode/Index.rst

@@ -0,0 +1,87 @@
+:navigation-title: Maintenance Mode
+..  include:: /Includes.rst.txt
+..  _maintenance-mode:
+
+=======================================================
+Maintenance mode: Prevent backend logins during upgrade
+=======================================================
+
+Set the backend into maintenance mode to prevent editors or even all
+administrators and CLI tools to access the TYPO3 backend.
+
+The maintenance mode is useful to prevent changes to the content during
+`Patch/Bugfix update <https://docs.typo3.org/permalink/t3coreapi:minor>`_
+and `Major upgrade <https://docs.typo3.org/permalink/t3coreapi:major>`_,
+database backups or in any other case where you want to prevent backend
+users from accessing the backend.
+
+..  note:: During maintenance mode the TYPO3 frontend works as usual. Depending
+    on your sites functionality, frontend actions might continue to alter
+    certain database tables or files in the file system.
+
+..  _maintenance-mode-total:
+
+Total shutdown for maintenance purposes
+=======================================
+
+A system maintainer can achieve total TYPO3 backend shutdown for maintenance
+purposes in module
+:guilabel:`Admin Tools > Settings > Configure Installation-Wide Options`
+by setting :ref:`[BE][adminOnly] <t3coreapi:confval-globals-typo3-conf-vars-be-adminonly>`
+to `-1`.
+
+It is also possible to add and remove this setting manually to the
+`additional.php <https://docs.typo3.org/permalink/t3coreapi:configuration-files>`_:
+
+..  code-block:: php
+    :caption: config/system/additional.php | typo3conf/system/additional.php
+
+    // Lock the backend for editors, admins and CLI are allowed
+    $GLOBALS['TYPO3_CONF_VARS']['BE']['adminOnly'] = -1;
+
+This setting excludes any user, including administrators like yourself from
+accessing the TYPO3 backend or using any console command in `vendor/bin/typo3`.
+Scheduler tasks will also not be triggered.
+
+A similar effect can be achieved by creating a flag file,
+:ref:`LOCK_BACKEND <t3coreapi:confval-flag-file-lock-backend>` via console
+command:
+
+..  code-block:: bash
+
+    vendor/bin/typo3 backend:lock
+
+The flag file prevents any backend access, even by an administrator, it does
+however not disable the console command tool and can therefore be disabled
+via command:
+
+..  code-block:: bash
+
+    vendor/bin/typo3 backend:unlock
+
+..  tip::
+    If you edit the :ref:`var/lock/LOCK_BACKEND <t3coreapi:confval-flag-file-lock-backend>`
+    file and put a valid URL into this file, users trying to log into the backend
+    are redirected to that URL instead of being shown an error message. You can
+    use this feature to show a custom maintenance message.
+
+..  _maintenance-mode-editors:
+
+Lock the TYPO3 backend for editors
+==================================
+
+To prevent an installation's editors from logging into the TYPO3 backend during
+maintenance, go to module
+:guilabel:`Admin Tools > Settings > Configure Installation-Wide Options`
+and set :ref:`[BE][adminOnly] <t3coreapi:confval-globals-typo3-conf-vars-be-adminonly>`
+to `2` if you additionally want to block console commands including scheduler
+tasks, set it to `1`.
+
+It is also possible to add and remove this setting manually in the
+`additional.php <https://docs.typo3.org/permalink/t3coreapi:configuration-files>`_:
+
+..  code-block:: php
+    :caption: config/system/additional.php | typo3conf/system/additional.php
+
+    // Lock the backend for editors while admins and CLI are still allowed
+    $GLOBALS['TYPO3_CONF_VARS']['BE']['adminOnly'] = 2;