Video/slide accessibility improvements

[hexylena]

During a discussion about Galaxy a11y, https://twitter.com/DogGeneticsLLC/status/1381999606268059650 it was pointed out that we should make access to the markdown more prominent. (Perhaps we can render a plain-text without the html, but with the title slide/followup/etc?)

• link to markdown from video page
• link to markdown from slides themselves ('view as plain text' on cover slide?)
• make a nicer view of markdown with the metadata templated out. (e.g. this content)
• check for alt-text for images since we have not been doing that on slides yet (yikes!)

Additionally this link was mentioned, useful for our workshop guidelines.
https://user2021.r-project.org/participation/accessibility/