Docs make presumption about download directory

[rhdtownsend]

A number of places in the docs presume that the download directory will be ~/Downloads (see, e.g., the instructions here. While that may be true on MacOS, it certainly isn't part of the Linux FHS. Suggest making the docs more download-agnostic. For instance, in the case of the Phoenix download instructions, why not have the user directly download into their chosen spectral library directory, rather than to ~/Downloads?

Review link: https://github.com/openjournals/joss-reviews/issues/6601