[DOC]: Flatten CUB API documentation section

[gonidelis]

Describe the incorrect/future/missing documentation

Our primitives are listed both under

CUB Docs Root
| -- CUB API 
      | -- Namespaces
            | -- cub

and under

CUB Docs Root
| -- CUB Modules 
      | -- Warp Primitives
      | -- Block Primitives
      | -- Device Primitives

... which is correct. Given that CUB Modules gives a hierarchy view of our CUB primitives.

But it's not right when you peek into our CUB API section. This section should be a long flat page with a list of all our public names (defines, primitives, typedefs etc.) where the user can ctrl+F and look up the name that they want to and redirect accordingly.

This is highlighted by the fact that the API hierarchy is convoluted:

CUB Docs Root
| -- CUB API 
      | -- Namespaces      <-- hm... ok namespaces. nice!
            | -- cub
                  | -- Classes
                  | -- Enums
                  | -- Functions
                  | -- Namespaces      <-- wait, what!? namespaces again? I just want to look up cub::DeviceReduce::Reduce :'(
                  | -- Structs

Task

Make the CUB API section a single flat page which includes all our public names in a web searchable way.