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

[jaimergp]

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.