Remove alt tag instruction from release post docs

Please provide a brief explanation for this change:

!106387 (comment 995594324)

The release post template adds the alt text tag using the yml file's title. The line telling the PM to do so should be removed from the handbook as it's confusing.

Please indicate the types of revisions being suggested for the Product Handbook (please check all that apply):

• Small improvement (typos, clarifications, etc.)
• Adding a new section
• Modifying existing section
• Documenting a new process
• Adding a new page or directory
• Other

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• Assigned to Milestone

Author Checklist

• Provided a concise title for the MR
• Provided a brief explanation for this change (Say why, not just what)
  • (Attach screenshots, Slack conversations, etc. as necessary)
• Indicated the types of changes included in this MR
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• Assigned reviewers for this change to the correct DRI(s)
  • 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 the changes affect team members, or warrant an announcement in another way, please consider posting an update in #product or #whats-happening-at-gitlab linking to this MR.
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