Add new social icon for Zulip

[kylewlacy]

Description

Closes #1952. This PR adds a new social icon for the chat platform Zulip. Here's how the new logo would look on the Starlight docs site (to the right of the Discord and GitHub logos):

I based the logo on the file zulip-icon-circle.svg from the Zulip repo. The repo is Apache 2.0-licensed, but based on zulip/zulip#13906, my impression was that the Zulip team didn't have a specific license for their logo but were open to usage when used for linking to Zulip itself (I will post in that issue to see if they would have any concerns about this PR as well, for good measure).

The original SVG itself featured the background circle and the inner "Z" as separate elements, so I basically just grabbed the "Z", scaled/positioned it, and saved it as its own SVG. I also tried having a negative "Z" within a circle, but felt that didn't read as well at a smaller scale.

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🦋 Changeset detected

Latest commit: 57b760c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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Name	Status	Preview	Updated (UTC)
starlight	✅ Ready (Inspect)	Visit Preview	Jun 8, 2024 6:14am

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[delucis]

Thanks for adding this @kylewlacy! Noticed on small opportunity for improvement, but otherwise this PR looks perfect 🙌

[delucis]

Thanks again @kylewlacy! Will get this released now.