feat: Validate author/authors frontmatter for blog posts

[markcheret]

Description

Add validation for author attribution in blog posts and contributor documentation.

Requirements

• Check for author or authors field presence
• Support multiple formats:
  • Simple string: author: "Jane Doe"
  • Array: authors: ["Jane Doe", "John Smith"]
  • Structured: authors: [{name: "Jane Doe", email: "jane@example.com"}]
• Validate email format if provided
• Warn if author field is empty or placeholder

Use Cases

• Blog posts requiring attribution
• Multi-author documentation projects
• Contributor tracking for open source docs

Suggested Implementation

• Required for blog posts, optional for docs (path-based)
• Support author references (e.g., author: @janedoe linking to authors database)
• Validate against known authors list if provided

Example Configuration

path_overrides:
  - path: "blog/posts/"
    frontmatter:
      author:
        required: true
        allow_multiple: true