Move all videos to YouTube, fix links

[rfay]

For a long time I put videos on Dropbox for some reason.

Now, thanks to @bmartinez287 we have DDEV Foundation channel, https://www.youtube.com/channel/UCgrpE4arppZ8Z9NROJlT73A

• Download the Dropbox videos and put them on DDEV Foundation channel. Relink in blogs as needed (but absolutely update the contributor training page)
• Move the youtube videos from randyfay channel if possible (and update links). I may not have the original recordings of those.
  • Docker Architecture
  • XDebug
  • Go Debugging
  • Setting up Go Dev Env