Rename file for probability introduction
Add references to bibliography and correct date for Casella and Berger
Add probability references to introductory section
Move introductory material to its own booklet

It seems useful to have a list of all of the potential supplementary
textbooks in one spot, so I moved the overall motivation and introduction
to the textbook into its own file.
Update probability references

- Mention Hall and Heyde (1980) for dependent probability theory
- Remove link to Pollard's 'User's guide' since it's not actually free
  online, just the first page of each chapter.
Start to add references for general writing and grad school advice
Remove extra 'item'
Remove list to other resources

It's too editorial; I'm just going to give it as a handout.
Adding introductory probability material.