feat: Validate tags and categories frontmatter

[markcheret]

Description

Add validation for tags and categories frontmatter fields to ensure proper content organization and discoverability.

Requirements

• Check for presence of tags or categories fields
• Validate tag/category count:
  • Minimum: 1-2 (configurable)
  • Maximum: 5-7 (avoid over-tagging)
• Validate tag format (lowercase, kebab-case recommended)
• Detect duplicate tags
• Warn about overly generic tags ("general", "misc", "other")

Use Cases

• Documentation sites with tag-based navigation (MkDocs, Docusaurus)
• Blog posts requiring categorization
• Content discoverability and filtering

Suggested Implementation

• Different requirements for blogs vs docs (configurable via path overrides)
• Support both tags array and comma-separated formats
• Include tag quality heuristics (length, format)

Example Configuration

frontmatter:
  tags:
    min_count: 2
    max_count: 5
    format: kebab-case
    forbidden: ["general", "misc", "other"]