Wrangling edits

[ConGibbs10]

• explain using select for reordering columns and renaming columns (differentiate between rename function)
• explain arrange function
• explain group_by with multiple grouping variables
• explain helpers: row_number, first, last, desc, everything, fill, n, case_when
• should I explain the selector functions for the select function (e.g. matches, contains, any_of, etc)?
• should I explain joining?