Book improvements

[siefkenj]

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

• [x] PRs for existing issues
• [x] Tag relevant words with \index and add an index to the book
• [x] 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)
  • [x] Row reduction and solving systems
  • [ ] Applications of row reduction: finding inverses, finding maximal linearly independent subsets
  • [x] Computing 2x2 and 3x3 determinants