Add new Web IDE & single file editor recording to Practical Handbook edits handbook

Why is this change being made?

In a coffee chat with @rclayman the question came up about a walkthrough video on editing the handbook. Since there is no recording for the new Web IDE yet, I took action to record a new video and show the possible paths to edit the handbook (single editor, Web IDE).

This MR adds the recording to the practical handbook edits handbook (you may guess it - done using the new Web IDE ;-)).

HD video is still rendering in https://youtu.be/DJo4bQdawSo I have added chapter markers for easier access.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
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  • 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
  • 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 affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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