Rename notebooks and move them to 'content' subdirectories

[tomdonaldson]

This is our initial attempt at reorganizing the workshop content.

• Cheat sheet ("CS_") QuickReference notebooks were moved to content/reference_notebooks.
• Use case exercise and solutions notebooks were moved to content/use_case_notebooks and renamed to indicate the nature of the use case (e.g., proposal_prep.md and proposal_prep_solution.md).

While trying out some different names, I noticed that for long-ish names the last part of the name can be lost in certain displays like the jupyter lab file explorer and in browser tab titles.

Another minor observation is that the <use_case_name>_solution.md files show up before their corresponding <use_case_name>.md file in the jupyter file explorers. This seems odd to me (and is different from the order I see with ls on my Mac).

[stargaser]

Since we named the subdirectory use_case_notebooks and not exercise_notebooks, I am in favor of adding _exercise, like candidate_list_exercise.md. Then I expect Jupyterlab would order the notebooks the way we want.

[tomdonaldson]

Since we named the subdirectory use_case_notebooks and not exercise_notebooks, I am in favor of adding _exercise, like candidate_list_exercise.md. Then I expect Jupyterlab would order the notebooks the way we want.

That makes sense to me. I made that change.

[stargaser]

The renaming looks good to me, and no one has raised objections. Does @bsipocz need to change the CI/html-building as part of this PR before it can be merged?