Fix singleton routes and add missing query parameters

[gaetansenn]

Fixes: directus/docs#431

[ComfortablyCoding]

While supported I wonder if we should document aggregate querying? In a singleton context it does not make much sense

[ComfortablyCoding]

Same as https://github.com/directus/openapi/pull/43/files#r2586004571, in a singleton context group does not make much sense

[kheiner]

Just a random person here - but the current documentation separates the singleton object as having a rest endpoint that doesn't exist /items/{collection}/singleton. I'm not familiar enough with swagger/openapi to know if it's possible to both have the correct endpoint represented and avoid the confusion of having aggregate/count for a singleton object because the singleton endpoint is the same as the endpoint for non-singleton items.

If the choices are:

1. Correct the rest endpoint but have the documentation explain that you can count/aggregate/group a singleton, despite it having no value.
2. Retain the documentation with a failing rest endpoint to maintain separation of parameters for count/aggregate/group is kept

I think 2 makes the most sense for most cases. Of course, the best case scenario is that it is possible to have option 3 where the endpoint is correct and only relevant parameters.