Resolve or comment the differences in `conda-envs` between platforms and Python versions

[michaelosthege]

PRs #4785 and #4779 created separate env files for Windows, because the m2w64-toolchain dependency is Windows-only and conda doesn't support OS-conditional dependencies in env files.

The 3 env files that existed beforehand for different Python versions have subtle differences in the versions, such as pinning numpy or pandas versions.
These differences should be either resolved or explained by a comment.

[michaelosthege]

Also, we should consolidate the install guides and conda-envs to a single source of truth.