Replace copy/pasted ImageFunction with imported SchemaContext

[delucis]

What kind of changes does this PR include?

• Changes to Starlight code

Description

• Closes docsSchema typings conflict with defineCollection typings #767
• Updates our frontmatter schema to import the SchemaContext type from astro:content.

[changeset-bot]

🦋 Changeset detected

Latest commit: a34e290

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]

size-limit report 📦

Path	Size
/index.html	9.42 KB (0%)
/_astro/*.js	19.25 KB (0%)
/_astro/*.css	9.55 KB (0%)

[delucis]

Hmm, seems to make tests hang here and also locally. Not a great sign!

[dreyfus92]

LGTM ✅