Netdata Agent: Backup restore documentation (#17006)

A document on how to backup and restore the Netdata agent for the public docs.
This works in a test environment and I've used it once in production so please check it over as it may need ammendments.

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Co-authored-by: Fotis Voutsas <fotis@netdata.cloud>

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+# Backing up a Netdata Agent
+
+## Introduction
+
+When preparing to backup a Netdata Agent it is worth considering that there are different kinds of data that you may wish to backup independently or all together:
+
+| Data type           | Description                                          | Location                                                                                                                |
+|---------------------|------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------|
+| Agent configuration | Files controlling configuration of the Netdata Agent | [config directory](https://github.com/netdata/netdata/blob/master/docs/netdata-agent/configuration.md) |
+| Metrics             | Database files                                       | /var/cache/netdata                                                                                                      |
+| Identity            | Claim token, API key and some other files            | /var/lib/netdata                                                                                                        |
+
+
+## Scenarios
+
+### Backing up to restore data in case of a node failure
+
+In this standard scenario you are backing up your Netdata Agent in case of a node failure or data corruption so that the metrics and the configuration can be recovered. The purpose is not to backup/restore the application itself.
+
+1. Verify that the directory-paths in the table above contain the information you expect.  
+
+   > **Note**  
+   > The specific paths may vary depending upon installation method, Operating System and whether it is a Docker/Kubernetes deployment.
+
+2. It is recommended that you [stop the Netdata Agent](https://github.com/netdata/netdata/blob/master/docs/configure/start-stop-restart.md) when backing up the Metrics/database files.  
+   Backing up the Agent configuration and Identity folders is straight-forward as they should not be changing very frequently.
+
+3. Using a backup tool such as `tar` you will need to run the backup as _root_ or as the _netdata_ user in order to access all the files in the directories.
+
+  ```
+  sudo tar -cvpzf netdata_backup.tar.gz /etc/netdata/ /var/cache/netdata /var/lib/netdata
+  ```
+  
+  Stopping the Netdata agent is mostly required in order to back up the _database files_ of the Netdata Agent.
+  
+  If you wish to minimize the gap in metrics caused by stopping the Netdata Agent, then you could have a backup job or script that uses the following sequence:
+  
+- Backup the Agent configuration Identity directories
+- Stop the Netdata service
+- Backup up up the database files
+- Restart the netdata agent.
+
+### Restoring Netdata
+
+1. Ensure that the Netdata agent is installed and is [stopped](https://github.com/netdata/netdata/blob/master/packaging/installer/README.md#maintaining-a-netdata-agent-installation)
+
+   If you plan to deploy the Agent and restore a backup on top of it, then you might find it helpful to use the [`--dont-start-it`](https://github.com/netdata/netdata/blob/master/packaging/installer/methods/kickstart.md#other-options) option upon installation.
+
+   ```
+   wget -O /tmp/netdata-kickstart.sh https://get.netdata.cloud/kickstart.sh && sh /tmp/netdata-kickstart.sh --dont-start-it
+   ```
+  
+    > **Note**
+    > If you are going to restore the database files then you should first ensure that the Metrics directory is empty.
+    > 
+    > ```
+    > sudo rm -Rf /var/cache/netdata
+    > ```
+
+2. Restore the backup from the archive
+
+    ```
+    sudo tar -xvpzf /path/to/netdata_backup.tar.gz -C /
+    ```
+
+3. [Start the Netdata agent](https://github.com/netdata/netdata/blob/master/packaging/installer/README.md#maintaining-a-netdata-agent-installation)