Netlify CMS MVC 3 - Adding field descriptions and ability to control filename

Why is this change being made?

This MR adds descriptions to topic fields in Netlify CMS. It also adds a feild the designates the data file name and ending URL path for topics page.

It also removes the "related videos" block from Netlify CMS because the component isn't ready to be added to these pages.

Related https://gitlab.com/gitlab-com/marketing/inbound-marketing/growth/-/issues/722

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)