Tidy data edit (#914)

* No need to load ggplot2, tidyverse is loaded

* Add motivation for why to pivot + minor edits

* Add an exercise on making a contingency table

* Use index.Rmd instead of rmd to make the build work

* Case study should be sentence case

* Make figure more legible and don't rely on color alone

* Streamline example + names_transform

* Potential pathway for more advanced pivot_wider

* Pivot even wider example + new unite exercise

* Update case study to use full feature pivot_wider

* Change the pregnancy exercise

* Fix chapter reference

* Remove century example since first 2 digits is not century

* Add caption to floating figure

* Align parens

* Fix referencing

* Add alt text to all chunks with plots or figures

* Remove unused figure (related to year -> century)

* Make subparts of exercises alphabetical

* Remove references to names_transform, use mutate()

* Fix code style

* Few more chp/sec reference fixes

* Put line break back

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