add Slack icon

[russbiggs]

What kind of changes does this PR include?

• Changes to Starlight code

Description

• Based on feature request in discussion Slack social link support #1071
• Add Slack social link icon

[changeset-bot]

🦋 Changeset detected

Latest commit: b8e689f

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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[astrobot-houston]

Hello! Thank you for opening your first PR to Starlight! ✨

Here’s what will happen next:

1. Our GitHub bots will run to check your changes.
   If they spot any issues you will see some error messages on this PR.
   Don’t hesitate to ask any questions if you’re not sure what these mean!

2. In a few minutes, you’ll be able to see a preview of your changes on Netlify 🤩

3. One or more of our maintainers will take a look and may ask you to make changes.
   We try to be responsive, but don’t worry if this takes a few days.

[vercel]

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Name	Status	Preview	Updated (UTC)
starlight	✅ Ready (Inspect)	Visit Preview	Nov 17, 2023 1:47am

[delucis]

Looks like that worked!

Thanks again for this PR and welcome to the Starlight contributors. If we trust GitHub, you are contributor number 100! 🥳