Cleaned bookdown code, typo fixes, new Tips & Tricks page

ModernDive 1.1.0

• Typo fixes and clarifying wording tweaks
• With a big assist from @mariumtapal, we cleaned and refactored all R Markdown code to make it easier for future bookdown users to understand. However all code seen by readers of the print edition has been left intact.
• Appendix C (online only):
  • Renamed from "Reach for the Stars" to "Tips and Tricks".
  • Added Appendix C.1 on most common data wrangling questions we've encountered, mostly written by @smetzer180