docs: README links to missing docs/external-services.md; .html listed as Coming Soon but already implemented

[Frankie-Xu]

Affected area

• Other
• Document parsing

Summary

Two documentation mismatches against main (checked 2026-08-25):

1. README links to docs/external-services.md in two places, but that file is not in the repo. docs/ currently has adr/, assets/, and pdf-profile-agent.html only.
2. README lists .html under Coming Soon, but HTML parsing is already implemented and advertised in the env examples.

Reproduction

1. Open README.md Quick Start (env notes) and Additional Guides. Both link to docs/external-services.md.
2. ls docs/ — the file is missing (GitHub 404).
3. README Supported Formats still has .html in Coming Soon:
   • [ ] .epub .html .xml
4. Contrast with the implementation:
   • apps/api/.env.example and apps/worker/.env.example set SUPPORTED_EXTENSIONS=...html,.htm
   • Worker routes .html / .htm in apps/worker/app/services/document_parser/orchestration/format_router.py to HtmlParseAdapter → parse_html() in formats/html/document_parser.py

Expected

• The linked external-services guide exists.
• Supported formats match SUPPORTED_EXTENSIONS and the worker router.

Proposed fix

Docs-only follow-up (no API / worker / deploy / env-default changes):

• Add docs/external-services.md from the two .env.example files and README (no invented keys, no real secrets or private endpoints).
• Move .html / .htm to the supported list; keep .epub and .xml in Coming Soon.