contributing docs: document gallery shortcode

[dholbach]

In https://github.com/fluxcd/website/pull/956 we added the gallery shortcode - it'd be a good idea to document it in contributor docs.

[theVJagrawal]

Hey @dholbach, can you please elaborate it more, I would like to work on this.

[dholbach]

Usage of the gallery shortcode is in https://github.com/fluxcd/website/pull/956/files#diff-ecfb308058142b2d07fc1d26094e767cbed6fef5c569a807c0f137d471440dd9R251

It's upstream documentation is at https://github.com/mfg92/hugo-shortcode-gallery

And it'd be great to mention and explain its usage in https://github.com/fluxcd/website/blob/main/content/en/contributing/docs/writing-docs.md

[VinayakRaoDikshit]

@dholbach I want to work on this issue, please assign it to me.

[dholbach]

#1477 improved the situation - thanks for the PR. It might be good to add an example in addition to that, e.g.

{{< gallery match="wego*.png" sortOrder="desc" rowHeight="150" margins="5"
            thumbnailResizeOptions="600x600 q90 Lanczos"
            previewType="color" embedPreview=true >}}

and explain that this only works for page bundles.

[FavourDaniel]

Alright, working on it now.