Re-add Tools and tips handbook: Add find/convert command for batch image resize

Why is this change being made?

The batch command was added to the tools and tips handbook, but it got merged in the same hour window when the handbook was migrated into its new location.

This MR copies the details gitlab-com/www-gitlab-com!124570 (diffs) which was approved already.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
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  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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