* The last html block wasn't formatting properly, it needed a blank line
  between the paragraph and the text.
* The ordered list wasn't generating the sequence, but always showing 1.,
  the inner content/examples needed to be indented for that to work.
* Mark some pieces as inline code for highlighting.
* Tweak a couple sentences.

Action View, Active Record and Active Storage use the 'What is
FRAMEWORK?' title for the framework introduction, the other frameworks
use 'Introduction'. The 'What is' form is more descriptive and better
for SEO.

This change makes the titles more consistent by using the 'What is
FRAMEWORK?' title for the other frameworks.

- Added guidance on issues with webpacker (making actiontext more beginner friendly)
- Clarify confusion about whether `content` attribute in database is needed.

Convert examples to use `form_with` instead of `form_for` or `form_tag`,
which have been soft-deprecated.  Also rename form variable in examples
from `f` to `form`, as exemplified by 8ff7ca5d.

gem 'image_proccessing' is required.

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