Add caption management to handbook video pages

Why is this change being made?

As my part of the contribution to the %14.0 retrospective, I'm trying to improve our handbook instructions around video content and accessibility:

• Zoom offers captions / live transcription, but you have to turn it on
• YouTube auto-generates captions for videos on our channel, but routinely mangles technical terms and names. A quick polish makes our video content much more professional-looking and accessible. (Not just for folks with hearing issues, but those for whom English is not their first language!)

My original intent was to just update the instructions for video content in the release posts, but when I did a search for video-related pages in our handbook, I found none of them addressed the importance of transcriptions and captioning.

(This is part of my quite-personal quest to help GitLab be more accessible to my best friend, a hearing-impaired GitLab user. We believe everyone can contribute—let's take "hearing ability" out of the equation!)

Related to #11907 (closed) and specifically this portion of my retro feedback: #11907 (comment 609852397)

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)