[DOC]: Create a CCCL developer guide

Developing CCCL involves a lot of conventions you have to be aware as a developer. Those conventions vary between different subprojects, but also a lot of commonalities (especially coming from libcu++) exist. It's hard to stay on top of everything, so I propose we should have and maintain a developer guide for our contributors and ourselves. This guide may be maintained in the CCCL repository or as a set of wiki pages.

We already have a [CUB developer guide](https://nvidia.github.io/cccl/unstable/cub/developer_overview.html), which we link from [CONTRIBUTING.md](https://github.com/NVIDIA/cccl/blob/main/CONTRIBUTING.md#developer-guides).

Issues asking for such a contributing/developer guide:

- #95
- #1585
- #2504

Instructions on how to build and test CCCL, or using devcontainers, may be moved into this guide or at least referenced from it.

This documentation should be intended for CCCL contributors/developers only, not for users. As such, it should contain a big disclaimer, that any documented behavior is not guaranteed to be stable and may change at any time and without warning.