Displaying multiple videos on a single webpage

Closes https://gitlab.com/gitlab-com/marketing/issues/1571

1. add margin to .video_container to be able to display multiple videos in sequence:

Before:

After:

2. Make it clear on the handbook

Explain how to use multiple video tags, and make it very explicit that one cannot edit the code block, otherwise videos may not render properly.

cc/ @axil if you can review/merge :)