docs(css): add troubleshooting note for static files and server restart (#1596)

[xcalibur73]

Summary of Changes

Resolves #1596.

In en/css/README.md, right after students add the link to �log.css and refresh the page (Figure 14.2), we added an explicit note covering the three common reasons why new custom styles might not reflect:

1. Restarting the development server: When new static directories (static/css/) are introduced while python manage.py runserver is already running, Django often needs a server restart (Ctrl + C then
   unserver) to detect the newly created path.
2. Hard refreshing the browser: Browsers frequently serve cached stylesheets; a hard refresh (Ctrl + F5 or Ctrl + Shift + R on Windows/Linux, Cmd + Shift + R on macOS) forces a fresh fetch.
3. Checking DEBUG = True: Verifying that DEBUG = True in mysite/settings.py so Django continues serving static files during local development.

Verification

• Markdown formatting verified.
• Follows the welcoming, beginner-friendly Django Girls tone and style guidelines.