[PLAY-878] Add CSS for Changelog Dates

[gavhuang]

What does this PR do?

• Runway
• In the changelog's markdown file, use h5 for the date below the title (see screenshots)
  • CSS is the same as the Caption kit
• Add changelog dates to 12.24 and 12.25 and update the Example Markdown Styling page with the h5 description
• Redcarpet adds a copy link on hover to all headers, but this is a quick ship story

Screenshots:

Add Changelog Dates

Add h5 Description to Example Markdown Styling Page

How to test? Steps to confirm the desired behavior:

1. Go to /changelog

• In the changelog file, add a h5 right below the h1 title.

2. Go to /guides/design_guidelines/example_markdown_styling

• View the added h5 description for changelog dates

Checklist:

• LABELS Add a label: enhancement, bug, improvement, new kit, deprecated, or breaking. See Changelog & Labels for details.
• DEPLOY I have added the milano label to show I'm ready for a review.
• TESTS I have added test coverage to my code.