Invalid images on training.play-with-docker.com

[Privat33r-dev]

In the Learn more section the images are referring to invalid URIs (404):
https://www.docker.com/sites/default/files/d8/styles/500px_6_4_ratio/public/2020-03/Compacted-light%20bkgr.png?itok=Y5Z2Iw79
https://www.docker.com/sites/default/files/d8/styles/role_icon/public/2020-01/Moby%20Group.jpeg?itok=kmD1NvVU

[Privat33r-dev]

In fact, the DockerCon was in 2020. According to archived picture.

Also the provided link to slack is invalid.

I suggest to remove sidebar section since it leads to invalid links and contains invalid images (I can open PR). Or at least to redesign it so it leads to valid events and slack.

Source file: https://github.com/play-with-docker/play-with-docker.github.io/blob/master/_includes/sidebar.html

[marcosnils]

@mikesir87 . Any chance you have a new dockr.ly link for the slack community?