Pixi: Improve advice given for next gen image formats

Currently we don't list out the resources - that is being tracked in #4666

However, we should change this advice to be:

Image formats like [AVIF](https://rawcdn.githack.com/AOMediaCodec/av1-avif/67a92add6cd642a8863e386fa4db87954a6735d1/index.html), and WebP often provide better compression than PNG or JPEG, which means faster downloads and less data consumption. [Learn more](https://web.dev/uses-webp-images/?utm_source=lighthouse&utm_medium=unknown).

Consider displaying all your `` components in AVIF and WebP formats while specifying an appropriate fallback (e.g. JPEG 2000, JPEG XR) for other browsers. [Learn more](https://amp.dev/documentation/components/amp-img/#example:-specifying-a-fallback-image).

This means we should no longer depend on PSI for the text.

@cramforce and @kristoferbaxter since they both have brought this up.