Add presentations

[CasperWA]

This mainly adds the recordings as well as links to or directly provides the presentation slides/material.

Minor updates:

• Use vMAJOR for GH Actions.
• One line per one sentence syntax throughout content.md.
• Disable markdownlint warnings concerning use of raw HTML.
• Remove redundant file in "root" of assets folder.

[CasperWA]

@rartino I've added a couple of headers here under the Speakers section. I think to move all this material under a new section that replaces Registration in the future.
Do you think this is the right way to approach this?
I was thinking to link to the slides that are provided online, having a local download link as well (say a "mirror"), and then also store the recordings here in the repo to be downloadable.
Perhaps we should put a more "official" license for all files in the repository prior to uploading anything?