Follow-up to #39594, which added CSS in order to select shell commands
sans prompts on triple-click.
This commit adds several bash code fences and prompts where they were
missing, and removes a few where they were inappropriate.
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.
Keep the example complex enough to showcase involving Action Mailers
to either: bounce and halt processing or finish the email processing
by asking for more information.
Closes https://github.com/rails/rails/pull/37736
[ Paul McMahon, Kasper Timm Hansen ]
