Split introduction.md into mdbook files
This branch:
- Splits introduction.md into a set of mdbook files (automated)
- Updates the links in those files to be correct (automated)
- Updates the links to the mdbook to be correct (by hand)
- Adds instructions to HACKING.md (by hand)
There is absolutely no cleanup to the text (yet)!
Does much of the rest of #66.
Edited by Nick Mathewson