Add check for threshold, warn if NA in predictors, and minor styling cleanup

[JamesHWade]

I love the idea of applicable! I'm interested in contributing, but I'll create an issue to discuss before I do much more.

In taking a peak under the hood, I saw a few areas for cleanup and tidying. This included:

• merging 0.R and applicable-package.R into a single file of globals and imports
• deleting docs/ and pkgdown/ folders from the repo and adding to .gitignore
• converting messages from rlang to cli in line with tidyverse styleguide

I also added some error handling:

• apd_pca now aborts if threshold is not in [0, 1)
• score now throws a warning if you try to use score.apd_pca with missing values in predictors

I did my best to break these changes up into individual commits.

Cheers!