chore: introduce the stable-4.x branch (#2888)

### Description

Related to libretime/libretime#2887
libretime/libretime@f6d57d5

versioned_docs/version-stable-4.x/README.md

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+---
+title: Welcome
+sidebar_position: 10
+---
+
+Welcome to the **LibreTime documentation**, you should find everything you need to know about LibreTime.
+
+## Getting started
+
+**Are you new to LibreTime? This is the place to start!**
+
+The documentation is divided into multiple parts:
+
+- the [Administrator manual](./admin-manual) provide guides and references to setup and configure LibreTime.
+- the [User manual](./user-manual) provide guides and tutorials for managers, and content creators to use LibreTime.
+- the [Developer manual](./developer-manual) provide guides to integrate LibreTime, or improve and contribute to LibreTime.
+
+## Getting help
+
+Having trouble? We’d like to help!
+
+- [:question: Try the Forum; it’s got answers to many common questions](https://discourse.libretime.org/).
+- [:bug: Report bugs with LibreTime in our ticket tracker.](https://github.com/libretime/libretime/issues)
+
+## History
+
+LibreTime is a fork of AirTime due to stalled development of the open source version. For background on this, see this [open letter to the Airtime community](https://gist.github.com/hairmare/8c03b69c9accc90cfe31fd7e77c3b07d).
+
+Airtime itself was based on the software called Campcaster. You can find more information about the [early days of Campcaster and Airtime in the repository](https://github.com/libretime/libretime/blob/main/LEGACY.md).

versioned_docs/version-stable-4.x/admin-manual/README.md

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+---
+title: Administrator manual
+---
+
+Welcome to the **LibreTime administrator manual**, you should find guides and references to setup and configure LibreTime.
+
+:::caution
+
+This documentation assumes that you:
+
+- have basic understanding of command line interfaces,
+- have a basic understanding of networking.
+
+:::
+
+:::tip
+
+Before following any steps, be sure that your system is up-to-date.
+
+:::
+
+## Install and configure
+
+- [:rocket: Install LibreTime](./install/README.md)
+- [:arrow_up: Upgrade from a previous install](./install/upgrade.md)
+- [:airplane_arriving: Migrate from Airtime](./install/migrate-from-airtime.md)
+- [:gear: Configure your installation](./configuration.md)
+- [:lock: Put your installation behind a reverse proxy](./install/reverse-proxy.md)
+
+## Advanced
+
+- [:warning: Setup automated backups](./backup.md)
+- [:heavy_check_mark: Use a custom authentication system](./custom-authentication.md)

versioned_docs/version-stable-4.x/admin-manual/_category_.yml

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+label: Admin manual
+position: 20

versioned_docs/version-stable-4.x/admin-manual/backup.md

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+---
+title: Backup
+sidebar_position: 10
+---
+
+## Create a backup
+
+This guide walk you though the steps required to create a full backup of your installation.
+
+:::info
+
+Remember to **automate** and **test** the backup process and to have it run regularly. Having an **automated** and **tested** restoring process is also recommended.
+
+:::
+
+:::caution
+
+Feel free to pick the backup software of your choice, but know that rsync and similar tools aren't backup tools. You could use [restic](https://restic.net/) or [borg](https://borgbackup.readthedocs.io/).
+
+:::
+
+### Backup the configuration
+
+On common setups, you need to backup the entire `/etc/libretime` folder.
+
+### Backup the database
+
+You need to backup the PostgreSQL database, which holds the entire data of your installation.
+
+Here is an example to dump your PostgreSQL database to a plain text SQL file:
+
+```bash
+sudo -u postgres pg_dump --no-owner --no-privileges libretime > libretime.sql
+```
+
+:::note
+
+We use the `--no-owner` and `--no-privileges` flags to ignore roles
+and permissions details about the database. This can be useful when restoring
+to database or role that have different names (e.g. `airtime` to `libretime`).
+
+:::
+
+Please read the `pg_dump` usage for additional details.
+
+### Backup the storage
+
+You need to backup the entire file storage, which holds all the files of your installation.
+
+The path to your storage is defined in the [configuration](./configuration.md) file.
+
+## Restore a backup
+
+### Install or cleanup
+
+If you are restoring a backup on a fresh system, we recommend that you first [install LibreTime](./install/README.md), and **stop before the [setup tasks](./install/README.md#setup)**.
+
+If you are restoring a backup on an existing system, make sure to clean the old **database** and **files storage**.
+
+### Restore the configuration
+
+Copy the backed configuration files back to the [configuration](./configuration.md) folder.
+
+If you are upgrading LibreTime, edit the configuration file to match the new configuration schema and update any changed values. See the [configuration](./configuration.md) documentation for more details.
+
+### Restore the database
+
+Restore the database by using the one of the following command depending on the format of you backup file:
+
+```bash
+# With a plain text SQL file
+sudo -u libretime libretime-api dbshell < libretime.sql
+
+# With a custom pg_dump format
+sudo -u postgres pg_restore --no-owner --no-privileges --dbname=libretime libretime.dump
+```
+
+:::info
+
+The `libretime-api dbshell` command is a shortcut to the `psql` command, and automatically passes the database access details (e.g. database name, user, password).
+
+:::
+
+:::note
+
+We use the `--no-owner` and `--no-privileges` flags to ignore roles
+and permissions details about the database. This can be useful when restoring
+to database or role that have different names.
+
+:::
+
+If you are upgrading LibreTime, make sure to apply the [database migration](./install/upgrade.md#apply-migrations).
+
+### Restore the storage
+
+Copy the entire backed file storage back to the storage path.
+
+The path to your storage is defined in the [configuration](./configuration.md) file.