[IMPROVEMENT] Improve 'recovery from longhorn backup without system installed' doc.

[khushboo-rancher]

Describe the bug
Need to change below in the 'recovery from longhorn backup without system installed' document.
https://longhorn.io/docs/1.0.0/advanced-resources/data-recovery/recover-without-system/

1. Elaborate 4th point of the document on entering BACKUP_URL - Indicate the location we can retrieve the backup name and volume name and then form the url. for example - In the backup.cfg file, these information exist in the beginning.

{"Name":"backup-580cb9538adb4da9","VolumeName":"volume-1","SnapshotName":"8adb8b45-4c13-4ff5-a2f5-a95b2cb4f35d" ...

2. Better/simple way of putting set Backup Target Credential Secret
3. A link to understand qcow2 vs raw format or get files out of a qcow2

Note: This should be taken care in sync with #1521

[nickramser]

This isn't 100% related to the updating the documentation and this issue, but I thought it was worth mentioning since I came across this issue and #1521 looking for a similar information. I created a separate container which can export longhorn backups out of the system and extract the files from qcow2 into a tgz per volume, https://github.com/nickramser/longhorn-backup.

Someone looking for updated documentation or updating the documentation could leverage some of the same logic to create a docker container/command for (1) and to have an example using nbd for getting files out of qcow2 for (3).

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• If labeled: require/doc Has the necessary document PR submitted or merged (including backport-needed/*)?
  The documentation issue/PR is at docs(backup): Add further information for 'recover-without-system' page website#580

[roger-ryao]

Verified website on master 20221121

• https://longhorn.io/docs/
• website master (e266055c)

The test steps

1. Check PR review conversation.
2. Verify the result on https://longhorn.io/docs/

Result Passed