Add pandoc installation to setup instructions

[sjspielman]

We need to have them install pandoc if we want to knit from terminal. Otherwise, update instructor notes to only knit from RStudio.

Originally posted by @sjspielman in https://github.com/AlexsLemonade/reproducible-research/issues/184#issuecomment-2299667390

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This issue tracks adding pandoc installs to instructions:

• For Mac, we'll have them use the GUI installer for their system: https://pandoc.org/installing.html#macos
• For Windows, they won't need a standalone pandoc since this pertains to CLI usage. So, for WSL 2, we can have them run apt-get install pandoc