Docs: guidelines for embedding videos

Problem to solve

Since we're adding support for embedding videos (https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/472, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29683), we want to create guidelines for:

• What kind of videos we want
• When to link them or embed them
• The maximum age of the video (to avoid outdated videos)
• Anything else?