Docs: 12-factor tutorial maintenance

[erinecon]

Overview

Mostly directory-related maintenance work spanning across all four 12-factor tutorials.

Summary of changes

• The "What you'll do" sections are now numbered lists.
• Any mention of the root directory (e.g., /django-hello-world) now contains a tilde in front (e.g., ~/django-hello-world).
• Captions on full code examples now contain captions with explicit reference to the full path (e.g., app.py is now ~/flask-hello-world/app.py). This change did not affect snippets of YAML files, which contain no caption whatsoever.
• Some mentions of visitors_app.py were renamed to app.py as the visitors_app.py naming was due to the multiple Python files needed for the spread tests.

[medubelko]

FWIW, for me, relative vs. absolute pathing doesn't aid in understanding the folder structure. My confusion comes from having two dirs with the same name differentiated by punctuation, one nested in the other.