Auto merge of #28371 - killercup:docs/trpl-markup-fixes, r=steveklabnik
Auto merge of #28371 - killercup:docs/trpl-markup-fixes, r=steveklabnik

- Headlines begin at 1st level now like the rest of the book
- All Headlines a blank line above and below
- Fix links in this chapter's TOC

r? @steveklabnik 

GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust