Add categories parameter to bank statements API and improve README

- Add optional `categories: Optional[List[str]]` parameter to
  `process_bank_statement_document` and `process_bank_statement_document_url`
- Include categories in the POST request body for transaction classification
- Add test coverage for the new categories parameter
- Rewrite README: add H1 title, TOC, all API sections (bank statements,
  checks, W-2/W-8/W-9, business cards, any docs, classify), error handling,
  contributing guide, and fix broken variable references and bare URLs

Made-with: Cursor

Author: dbirulia