Enhancement: Allow the `contentRelationship` field to have a quick search feature

[estruyf]

Based on feedback received in the documentation: FrontMatter/feedback#5 (comment)

It would be useful to have a search/filter ability in the contentRelationship field.

[estruyf]

Check if we can make use of https://ui.shadcn.com/docs/components/combobox

[estruyf]

Implementation is done with https://headlessui.com/react/combobox

[estruyf]

@aluciu, the ability to filter your content has now been added to the contentRelationship field. Feel free to give it a try in the latest beta version.