[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

Beta Give feedback

1. Write announcement for the merging of the repos
2. Merge git histories into cccl repo #41
   infrastructure +1
   
3. New cmake infrastructure #43
   27 of 28
   infrastructure +1
   
4. Transfer issues/PRs from old repos #42
   6 of 9
   +0
   

Future Tasks

Beta Give feedback

1. [EPIC] GitHub Actions CI #68
   12 of 14
   infrastructure +1
2. Archive old repos #1181
   +0
   

Monorepo Docs

Beta Give feedback

1. [FEA]: Update the README #290
   +0
2. [DOC]: Prepare summary of CMake changes to help users transition to the monorepo #212
   cub doc infrastructure libcu++ thrust +5
3. Add contributor guide #499
   +0