I wanted to add a custom bang for the FastGPT feature and I didn't find the custom bangs feature where I would expect it (near the search bang / quick bangs section). I found it afterwards by doing a kagi search to find it in the official docs.
It would make sense to me to have a link to custom bangs (https://kagi.com/settings?p=custom_bangs) on the Search tab, near the existing search bang toggle and configuration.
