Narrowing Page

[gcrev93]

I noticed most pages have a mini table of contents and an introduction at the top. Narrowing has subtopics in the outline but it is not present at the top of its page, nor is an introduction.

Subtopics:

Also the never type and Exhaustiveness checking subsections are empty: is this correct?

(If you would like me to separate these into different issues I can. Just wanted to keep all notes for the page together)