feature/add supplements: Adds first-class support for "supplement" pages

[heroheman]

Description

Adds first-class support for "supplement" pages from the SCP Wiki, enabling automated crawling and post-processing of supplementary content.

Changes

• New Item Type: ScpSupplement class in items.py
• New Spider: ScpSupplementSpider to crawl pages tagged with supplement from https://scp-wiki.wikidot.com/system:page-tags/tag/supplement
• Post-Processing: run_postproc_supplement() command that:
  • Extracts parent SCP references (e.g., "SCP-6881" from link)
  • Extracts parent tale series (e.g., "planetfall" from planetfall-7)
  • Processes history, images, and hub references
  • Outputs to data/processed/supplement/ with index.json and content_supplement.json
• Makefile Integration: Supplements now included in scp_crawl and scp_postprocess targets
• Documentation: Updated README with supplement spider usage and output structure

Output Structure

{
  "planetfall-7": {
    "title": "Planetfall: 7",
    "parent_scp": null,
    "parent_tale": "planetfall",
    "created_at": "2023-05-15T14:30:00",
    ...
  }
}

Testing

make supplement              # Crawl + post-process
scrapy crawl scp_supplement  # Crawl only