spring case study chapter (#360)

* edits the introductory sections

* tweak list spacing

* edits to the new stat section

* edits the geom section

* expands grid tutorial

* small changes to grob sections

* edits part 4

* starts editing the scales section

* edits the scales section

* fixes df/some_data references

* Minor tweaks

* minor fixes

* adds discussion of gtable

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Co-authored-by: Hadley Wickham <[email protected]>