Update following June 2021 Showcase

[datajoely]

Description

• Remove references to Discourse, introduce Discord and GitHub Discussions
• Add slides to repo

Checklist

• [x] Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• [x] Assigned myself to the PR
• [x] Added tests to cover my changes

Notice

• [x] I acknowledge and agree that, by checking this box and clicking “Submit Pull Request”:

• I submit this contribution under the Apache 2.0 license and represent that I am entitled to do so on behalf of myself, my employer, or relevant third parties, as applicable.

• I certify that (a) this contribution is my original creation and / or (b) to the extent it is not my original creation, I am authorised to submit this contribution on behalf of the original creator(s) or their licensees.

• I certify that the use of this contribution as authorised by the Apache 2.0 license does not violate the intellectual property rights of anyone else.

[datajoely]

Also added this table + @yetudada 's PyCon talk