Add new vignettes for pkgdown

[stufield]

• building out the articles for the pkgdown website
• new articles on safe processes
• train and test data splits
• beginning the FAQs pages

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1114fcc) 73.61% compared to head (99fee8d) 73.61%. Report is 4 commits behind head on main.

:exclamation: Current head 99fee8d differs from pull request most recent head bee0198. Consider uploading reports for the commit bee0198 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #64 +/- ## ======================================= Coverage 73.61% 73.61% ======================================= Files 39 39 Lines 1440 1440 ======================================= Hits 1060 1060 Misses 380 380 ```

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