Add zulip and deduplicate BlueSky

[RRosio]

I noticed that Zulip was missing from the Jupyter website and thought it could be added as the community is quite active there. I was not sure if it should be added elsewhere like the 'Follow Us' section, but I can add it there if it should indeed go there!

I also noticed the BlueSky sandwich mentioned by Paul in #780 (comment) so I thought it might work to update it here 🙂

[ivanov]

Had to resolve a conflict, since Matthias already deduplicated on the main branch directly.

[ivanov]

pinging @jupyter/media-strategy-working-group (which I just created) to make sure they're aligned with this change.

[andrii-i]

Hi everyone. Thank you for working on this.
There's a "Audit with Lighthouse" test failing, is this problematic?
Other than that, this LGTM.

[choldgraf]

I think this is a good improvement. We might always improve this further but I think this is good enough to just merge as is. This is not the cause of lighthouse failures for the test so I think that's fine