Fix image integration README

[delucis]

Changes

When the image integration README is pulled into the docs site, Astro tries to render <Image /> and <Picture /> which are then undefined, breaking the build. See withastro/docs#1457

I’ve also wrapped a few more snippets of code in backticks where it makes sense.

Testing

n/a

Docs

All docs, all the time 🕶️

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[sarah11918]

Woo! Thanks for catching my lack of backticks, @delucis 🙌 LGTM after changed in the other spot, too!