In the Action Text guides, `app/assets/stylesheets/actiontext.css`
is specified as the file used to style the Action Text editor and
content but the actual file generated from `rails action_text:install`
is `app/assets/stylesheets/actiontext.scss`.
This change simply corrects the file extension shown in the guide.
- Move some actiontext/README.md content to Action Text Overview guide
- I added WIP label to that guide since we definitely want to complement it.
- Add Action Text to Major Features of Rails 6.0
Similar approach was used in #34812