Add API wrappers, tests, and comprehensive README with response examples

• Add DownloadApi class (inherits RenderApi for backward compat), AuditFeesApi,
  EdgarIndexApi, and 3 new FormAdvApi methods (get_other_business_names,
  get_separately_managed_accounts, get_financial_industry_affiliations)
• Update Download API endpoint from archive.sec-api.io to edgar-mirror.sec-api.io
• Add integration test suite (40 tests) covering all API classes
• Add collapsible response examples to all README sections with full example
  response links, matching the NPM package structure
• Split Form ADV, Insider Trading (Form 3/4/5), and Form N-PX into sub-sections
• Add Bulk Datasets section, Quick Start code, and complete TOC with verified links
• Add examples/api-responses/ with JSON response files for all APIs