[Internal Cleanup] Look into consolidating .gitignore files

See https://github.com/NVIDIA/cccl/pull/3201#discussion_r1894041457:

Q: Is it possible that we consolidate `.gitignore` files at the root directory and not have independent ones per sub dir...?

Quick overview:

```bash
git switch main
git rev-parse HEAD
```
```
86b91186edf664976fa3cbe4c10c602be5ee7465
```
```bash
find . -name .gitignore
```
```
./thrust/.gitignore
./.gitignore
./cub/.gitignore
./cub/benchmarks/docker/.gitignore
./cub/examples/device/.gitignore
./cub/examples/block/.gitignore
./cub/test/.gitignore
./python/cuda_cooperative/.gitignore
./python/cuda_parallel/build/temp.linux-x86_64-cpython-312/_deps/nvtx-src/.gitignore
./python/cuda_parallel/build/temp.linux-x86_64-cpython-312/_deps/catch2-src/.gitignore
./python/cuda_parallel/.gitignore
./python/cuda_parallel/.pytest_cache/.gitignore
./benchmarks/scripts/.gitignore
./.pytest_cache/.gitignore
./.ruff_cache/.gitignore
./docs/.gitignore
./docs/tools/repoman/omni/repo/format/.gitignore
./libcudacxx/.gitignore
./libcudacxx/codegen/.gitignore
./libcudacxx/test/.gitignore
./libcudacxx/test/utils/nvidia/getsm/.gitignore
./.mypy_cache/.gitignore
```

Start with the root .gitignore and compare it with the nested .gitignore files listed by the issue’s find command, checking which patterns are duplicated or depend on directory scope. Use git check-ignore to verify representative paths after any consolidation. Done means the ignore behavior is preserved while the independent project-owned files are consolidated where practical.

Written by the indexing model from the issue text.