Create README for the Flavian theme with pattern catalog

[PAMulligan]

Description

The themes/flavian/ directory has no README file. New contributors have no quick way to understand the theme's purpose, what block patterns are included, or how to customize it.

Task

Create a themes/flavian/README.md that documents:

1. Theme Overview — what Flavian is and its design philosophy
2. Block Patterns — list all patterns in patterns/ with one-line descriptions
3. Theme.json — brief overview of the design tokens (colors, typography, spacing)
4. Customization — how to modify colors/fonts via theme.json
5. File Structure — layout of the theme directory

Files to Create

• themes/flavian/README.md

Files to Reference

• themes/flavian/theme.json — design tokens and settings
• themes/flavian/style.css — theme metadata header
• themes/flavian/patterns/*.php — all block patterns (check each file's header for Title/Description)
• themes/flavian/templates/ — FSE templates
• themes/flavian/parts/ — template parts

Acceptance Criteria

• README exists at themes/flavian/README.md
• All block patterns listed with their title and one-line description
• Theme.json color palette and font families mentioned
• Clear, well-organized markdown formatting
• No hardcoded values — reference theme.json tokens where applicable