Add a section to data docs about the metadata attribute used by plugins

[stichbury]

In working on #2888 @astrojuanlu made the comment that we needed to add some content about the metadata attribute (#2537) that can be used by plugins, such as the preview stuff for viz? We do need this in the data_catalog.md file that introduces settings in the catalog YAML. @merelcht says "I don't think there's any existing docs other than the part about "preview" datasets in viz. I'd suggest doing that in a separate task because it requires new content".

This is that ticket 👯‍♀️

[merelcht]

I just realised there was already a ticket for this: kedro-org/kedro-viz#1650

[astrojuanlu]

Indeed, let's close in favor of that one.