[DOC]: Unify Docs hierarchy tree

[gonidelis]

https://nvidia.github.io/cccl/cpp.html#cccl-cpp-libraries

Should not redirect to three different documentation pages but maintain the hierarchy on the left side every time we unfold one of the 4 existing C++ libraries.

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As part of this issue we should take care to remove the CUB prefix from every section in the cub documentation tree.

• Make CCCL Docs landing page the root for the other three libraries plus any extra material.
• Remove CUB prefix from every top CUB documentation section.

[bernhardmgruber]

This seems to have overlap with #2863 which tries to "Find a way for the top-level ToC to persist."

[gonidelis]

It's a niche point where our work crosses paths. I 'll work with Allison. Thanks!