Create tutorial per issue 37

[monicathieu]

Example code is there! Note that it doesn't run yet! I want to try to figure out a longer-term fix for the "back-end" issue I have noted in #39 before I actually try to check the example code for runnability.

Once the back-end issue is fixed, and we can jump back into writing more complex code examples, I also want to add an example on this page for creating some kind of a carpet-plot for visually inspecting data completeness/missingness within subject but across timepoints. For example, a multilevel regression with timepoint as a within-subjects predictor variable will be thrown off when not every subject has data at every timepoint. People will likely want to visualize patterns of partial data by timepoint in order to make judgments about whether to drop subjects with incomplete data or to impute missing data, so it'll be good to show people a means of visually assessing data completeness to aid in that decision.

I'm creating this pull request now to track discussions about this tutorial, but we don't need to merge it until that carpet-plot example is added.

[monicathieu]

Currently trying to set up the deploy-book GitHub Action workflow to run whenever edits are made on pull requests associated with branches called issue-**. Currently I need this to test the viability of the R environment setup GitHub actions. Eventually, we might want it to check buildability of contributed tutorials.