The redox book contains some broken links throughout the various
markdown pages. Fix all broken links (to existing pages) such that
the documentation can be used as intended.

The changes include:
- Editing for flow and understandability.
- Unifying how links are included in the source files.
- Adding an `About this Book` page to the introduction.
- Adding a few examples or new pieces of information I felt were
  lacking when I read the book.
- Unifying 'user space' and 'kernel space' spelling.
- Added myself to the authors list in `overview/welcome.md`.