Document image crate integration on docs.rs

[Shnatsel]

Right now the image feature in jxl-oxide crate is disabled on docs.rs. Because of that the docs on the integration do not show up on docs.rs at all. This PR fixes that.

Details

doc_auto_cfg is a nightly rustdoc feature, so it's only enabled for docs.rs to make it possible to build the docs for this crate on stable. All ways of documenting conditional features are currently nightly-only, but are also widely used so they're unlikely to outright break.

You can preview the generated documentation with this command: cargo +nightly rustdoc --open --features=image -- --cfg=docsrs

It looks like this:

[tirr-c]

Ooh I've absolutely missed that, thanks!

[Shnatsel]

A point release including this would be very welcome, so that I could promote the integration on social media and link to the docs.rs page directly.