Release post handbook - updates for tech writing review

Why is this change being made?

• Bring the checklist format we had before: one checklist item for each task.
• Add back the most important tasks to be checked by technical writers, item by item.
• Better instructions on how to add videos.
• Instruct the TW reviewer to set the release-post-MR dependent on the docs-MR and feature-MR.
• Update the structural review of the release post handbook.
• Closes #6672 (closed).

Preview:

• Structural check: https://rp-handbook-updates-tech-writing.about.gitlab-review.app/handbook/marketing/blog/release-posts/index.html#structural-check
• Release post item - MR description template: https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/rp-handbook-updates-tech-writing/.gitlab/merge_request_templates/Release-Post-Item.md#tech-writing-review

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what