[Docs]Document support for Markdown, HTML, custom CSS, and custom partials/shortcodes

[Vincamine]

Current State

Desired State

We should ensure our documentation clearly states:

• Both Markdown and raw HTML are supported in content files
• How to apply custom CSS: Inline (inside Markdown/HTML) and Separate included via a partial in layouts
• How to create and use a custom partial (for layouts)
• How to create and use a custom shortcode (for content authors)

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Contributor Guide and Resources

• 📚 Instructions for contributing to documentation
  • Layer5 documentation site and source
• 🎨 Wireframes and designs for Layer5 site in Figma (open invite)
• 🙋🏾🙋🏼 Questions: Layer5 Discussion Forum and Layer5 Community Slack