docs: fix outdated astro:assets imports and snippets

[Nloperena]

Description (required)

Docs-only fix for outdated astro:assets imports and an incorrect <Image /> snippet, matching the current public API.

• Removed matchHostname, matchPathname, matchPort, matchProtocol, and emitImageMetadata from astro/assets/utils (removed in fix(astro): remove unused re-exports causing Vite build warning astro#16197 and Recipe idea: sending email from an Astro app (and testing it locally) #14306).
• Moved propsToFilename and hashTransform docs from astro/assets/utils to astro/assets (fix: update Cloudflare adapter's default image service astro#15435).
• Documented background on ImageTransform / getImage() instead of as a typed <Image /> prop.
• Updated the v6 upgrade guide so it no longer recommends emitImageMetadata() after emitESMImage() was removed.

English only.

References

• Closes Incorrect snippet and outdated imports in astro:assets docs #14281

[astrobot-houston]

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File	Note
en/guides/upgrade-to/v6.mdx	Source changed, localizations will be marked as outdated.
en/reference/modules/astro-assets.mdx	Source changed, localizations will be marked as outdated.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.

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[ArmandPhilippot]

Thanks, but AI generated PR are not helpful. If you read the issue, you'll see we are trying to figure out what is the best course of action here. Nothing has been decided yet. So, I'm closing this.