Book improvements

[siefkenj]

There are several ways in which the current textbook should be improved:

• PRs for existing issues
• Tag relevant words with \index and add an index to the book
• Integrate tutorial worksheets as an appendix
• Revise based on Perusall feedback
• Complete solutions for existing exercises
• Add exercises to some sections (decide which ones)
• Cross-reference modules with learning-objectives list
• Add appendices for missing content (currently in the videos but not the textbook)
  • Row reduction and solving systems
  • Applications of row reduction: finding inverses, finding maximal linearly independent subsets
  • Computing 2x2 and 3x3 determinants