docs: fix outdated astro:assets imports/snippets

[sharadvc]

Summary

Update the astro:assets reference for current exports:

• Drop removed astro/assets/utils helpers from imports/docs
• Document propsToFilename / hashTransform under astro/assets
• Fix <Image /> background snippet frontmatter
• Document background on ImageTransform

English page only (keeps the PR small). Upgrade-guide emitImageMetadata wording left for a follow-up.

Fixes #14281

[astrobot-houston]

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[ArmandPhilippot]

As I said in #14529 this is an unwanted PR: the issue is labeled with should this be documented?. This is a big clue that there is nothing actionable yet. And if you had read the issue, the first part is already fixed.

So, I'm going to close this as well as your other PRs because you haven't listen me the first time:

• docs: fix outdated astro:assets utils and Image background example #14529
• docs: clarify node adapter middleware next() and Fastify error pages #14530

Please refrain to submit automated PRs here and do not ignore maintainers comments.