feat: Add dark and light variants for images

[dragomano]

Description

This PR adds dark and light variants for images in Markdown.

Source of inspiration: the corresponding feature in Material for MkDocs

Examples:

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🦋 Changeset detected

Latest commit: 10d29fe

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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Tracked Files

Locale	File	Note
en	guides/authoring-content.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.