[astro-purgecss] Handle custom 404 page

[Jaanilj]

Description

This PR adds special handling for the 404 custom error page in Astro integration, ensuring proper file path generation based on Astro's documentation standards.

Changes

• Added dedicated handling for 404/ pathname to generate 404.html at the root level
• This follows Astro's custom 404 error page convention as documented at https://docs.astro.build/en/basics/astro-pages/#custom-404-error-page

Testing

• Tested with 404.astro page in the src/pages/ folder

Checklist

• Create changeset
• Code follows project style guidelines
• Documentation has been updated if necessary
• All tests pass
• Commits are properly formatted and descriptive
• PR targets appropriate branch

[changeset-bot]

🦋 Changeset detected

Latest commit: b6eb919

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

This PR includes changesets to release 1 package

Name	Type
astro-purgecss	Minor

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[mhdcodes]

@Jaanilj Thanks for taking the time to send this fix. If this is final, I’ll go ahead and merge it.