Throw error if a user’s project adds MDX or sitemap integrations

[delucis]

What kind of changes does this PR include?

• Changes to Starlight code

Description

• Users could potentially run into cryptic errors if they themselves added the MDX or sitemap integrations that come bundled with Starlight.
• This heads that off by throwing a clearer error if we detect it.

[changeset-bot]

🦋 Changeset detected

Latest commit: 66bf5f1

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	Minor

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[astrobot-houston]

size-limit report 📦

Path	Size
/index.html	9.42 KB (0%)
/_astro/*.js	16.33 KB (0%)
/_astro/*.css	8.7 KB (0%)