[Internal Cleanup] Look into consolidating .gitignore files

[rwgk]

See #3201 (comment):

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

Quick overview:

git switch main
git rev-parse HEAD

86b91186edf664976fa3cbe4c10c602be5ee7465

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