Draft: Geo: Document backups --s3tool awscli option for both manual and cron run jobs

What does this MR do and why?

The main points that we should highlight in the documentation are:

• We can override the default of s3tool by passing the --s3tool flag with awscli option. We can provide some use cases for this.
• This can be used for both manual and cron jobs.
• This is not intended to work with MinIO; we can track the progress of this capability following gitlab-org/charts/gitlab#3157 (closed).

This MR updates the documentation according to the points above.

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Related to #437688 (closed)