CEP: Draft meaning of `~/.conda/environments.txt`

Key points:

- This is `conda`-specific, other tools in the ecosystems might not want to use it
- `conda` can choose when to register envs (e.g. temporary ones used internally are not registered), but the user cannot disable that (see `context` for available hidden option)
- Its location is user-specific, which is controversial (what about multiuser installations? who owns the registry in that case?). Need to clarify the shortcomings and propose strategies forward.