Improve shortvid

[HugoGresse]

• add download button on teaserVideo and for image in side panel
• add teaserImageUrl
• generate video & image with ShortVid
• improve readme

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit d3ec668):

• https://conferencecenterr--pr105-improve-shortvid-8tvzmpdh.web.app
• https://apiopenplanner--pr105-improve-shortvid-9j8gch4c.web.app

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