Documentation for MySQL Shell backup engine

[rvrangel]

This PR adds the documentation needed for the new MySQL Shell Backup engine that is being added in https://github.com/vitessio/vitess/pull/16295

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[shlomi-noach]

Direct preview links to changed docs:

• https://deploy-preview-1846--vitess.netlify.app/docs/21.0/user-guides/operating-vitess/backup-and-restore/creating-a-backup/#using-mysqlshell-experimental
• https://deploy-preview-1846--vitess.netlify.app/docs/21.0/user-guides/operating-vitess/backup-and-restore/overview/

[rvrangel]

@shlomi-noach @deepthi is there anything missing you would like me to cover here?

[shlomi-noach]

@rvrangel this looks good to me. Only did a brief review a couple days ago as the PR is still Draft - sounds like it's ready for a review now?

[rvrangel]

ah yes, marked it as ready for review now