Is your feature request related to a problem? Please describe.
Sometimes a resource will only ever be singular, for example you might have a top-level 'Site' that has site-level config. However this can only be represented as a collection. In this case it makes no sense for a collection of 'sites' as there is and will only ever be a single site

Describe the solution you'd like
It would be great to be able to define a singular / singleton collection, either in a separate category in the sidebar, or so that clicking on its type in the sidebar opens it for editing. This could be done in the collection config via an isSingleton flag.

Describe alternatives you've considered
The current workaround is to have a collection with a single item in it, however this is confusing for users of the CMS.