Move CSS comments docs to CSS directory

Move CSS related documentation to all live in the `/css` directory

Status: **Opened for visibility**
Reviewers: @kpeatt @ry5n 
JIRA: n/a

## Changes
- Comment docs were in a `css-comments` directory, now it's in `css/comments`

## To Do
- [x] Add an index to the main comments doc
- [x] Decide the folder/file structure (x/x.md vs just x.md) - _leaving as is (x/x.md)_
- [x] Implement folder/file structure - _leaving as is (x/x.md)_