[EPIC] Set up CCCL monorepo

[jrhemstad]

In order to unify and streamline both the development and use of Thrust/CUB/libcu++, we are going to merge them into https://github.com/NVIDIA/cccl.

Intended Outcome

Archive the existing repositories and have everything on nvidia/cccl.

Plan

• Get CI matrix up and running on top of new cmake infrastructure w/ submodules (Monorepo workflow #99)
• Merge Monorepo workflow #99
• Swarm to make CI matrix green across the board (Enable full matrix to complete successfully #102)
• Pause merging any new PRs to old repos
• Remove submodules and merge the combined git histories into nvidia/cccl (Merge git histories into cccl repo #41)
• Move all development to monorepo
• Transfer PRs/issues from old repos

2.2 Tasks

• Write announcement for the merging of the repos
• Merge git histories into cccl repo #41
• New cmake infrastructure #43
• Transfer issues/PRs from old repos #42

Future Tasks

• [EPIC] GitHub Actions CI #68
• Archive old repos #1181

Monorepo Docs

• [FEA]: Update the README #290
• [DOC]: Prepare summary of CMake changes to help users transition to the monorepo #212
• Add contributor guide #499