Clarify the dependency on `conda` and the requirement to create `conda.yaml`

[jklaise]

The dependency on conda is mentioned here, however I believe this could be made more explicit.

In particular, it is currently not clear that the user is expected to create a conda.yaml file using the appropriate commands before packing the environment via the tempo.serve.loader.save command.

On the same note, when I issue a save(model) command, even without a conda.yaml file an environment is packed, does it mean that in the absence of a conda.yaml everything in the current conda environment is packed?