Docs add delete switch to command rsync's usage

What does this MR do?

Adds --delete flag to the rsync command in section Storing Git data in an alternative directory

A pattern from customer tickets is that the current command is used repeatedly to help reduce the cut-over time, using --delete is necessary for the final rsync to ensure that the Git project files are a true representation of the current state.

Using it from the beginning removes the step of remembering to do this for the last usage and doesn't affect the intended workflow.

Related issues

Customer ticket ZD - internal link only

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• [TBD] Consider taking the GitLab Technical Writing Fundamentals course
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