Include additional guidance for creating deprecation post entries

Why is this change being made?

The current handbook guidance on creating deprecation post entries indicates that there should be a single deprecation post entry or .yml file per merge request.

Please set up one deprecation per MR by creating a new .yml file in the /data/release_posts/unreleased/ folder with the following contents:

However, I have noticed that other product managers are being more efficient and creating one MR that includes multiple deprecation post entries and .yml files.

As such I am proposing this modification to the guidance so that it is clear that this is an option.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • 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 in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)