Summarize the lecture of ty into a chapter (#530)

* Summarize the lecture of ty into a chapter

* Add note that def-id is explained later

* Add mark-i-am fixes

* Address some of Niko's comments

* address last review comments

* fix link

Co-authored-by: Who? Me?! <mark-i-m@users.noreply.github.com>

Author: 2 people