Update Open Source AI Workshop website now that event is over

Hi -- Now that the event is over, we need to update it to remove the "Register" button in the header.

We could perhaps wait to see when the YouTube recordings will be available, since the site will nee to be updated to add that info too.

I'm not sure we have discussed this, but maybe for a site like this, the "Register" button could be changed to a button that says something like "Watch Recorded Talks" and links to the YouTube page?

@gaelblondelle @Ioana240 WDYT?