Expand intro page and instructor notes

[ewallace]

It would be good to expand the intro page to give more on:

• what is snakemake
• why use snakemake
• the target audience

There's good examples of this in other carpentries lessons that could be quickly modified, e.g. workflows with nextflow. And that also has extra background and resources in instructor notes.

This suggestion is designed to make it more inclusive both to learners trying to choose whether to register for a course, and to those trying self-study.