Display Zulip and Discourse more prominently on the website

[choldgraf]

The Jupyter Discourse and Zulip are the two main cross-project spaces for async and sync discussion, respectively. However they're not easily discoverable from the Jupyter website, which is a missed opportunity to encourage others to participate in a low-barrier way.

Currently these two pages seem to be the primary place for this information:

• The community page leads with a link to the Jupyter GitHub, which is probably not the best place for newcomers to get started. What if we added a section to the top of the page that lists both the Discourse and the Zulip with logos for each, and suggests they are the best places to start conversations with the broader community. We can leave the list of other links below as-is (minus the Discourse since it will be above)
• The social page lists the Discourse, but not the Zulip. I'd suggest we add Zulip there more prominently at the top (underneath discourse?) and consider adding logos for each of them so that it pops out visually more.

I'd also recommend that we add a sentence like the following to help others know what kinds of conversations to have in each space:

What's the difference between the Discourse and the Zulip? These two spaces serve complimentary purposes. The Jupyter Discourse is for asynchronous conversation that is easier to reference and update over longer periods of time. Use it for things you'd like more structured or deeper discussion around. The Zulip is for synchronous and quicker conversation and chat. Use it to quickly connect with others across the project. Zulip conversation can be messier and more chaotic because it's inherently more short-lived.

This could be a low-cost way to grow the visibility of these spaces, as well as a way to highlight the contributions that both Discourse and Zulip are giving to the project, since we're on "free open source" plans of both of them.