Bootstrap text-forge: composite action + migration from whattodo pipeline

[bongiozzo]

Goal: move the technical pipeline (combine → normalize → render → publish) out of bongiozzo/whattodo while keeping text/ and its history canonical.

Acceptance criteria

• Composite GitHub Action exists in this repo (tagged v1).
• A content repo can call it from a tiny workflow and publish GitHub Pages.
• Outputs produced match current whattodo behavior:
  • build/text_combined.txt
  • build/pandoc.md
  • build/text_book.epub
  • public/ru/ site build (MkDocs)

Work items

• Copy/port logic from whattodo:
  • scripts/mkdocs-combine.py
  • scripts/pymdown-pandoc.lua
  • epub/book_meta.yml + epub/epub.css (+ cover handling)
  • MkDocs hook mkdocs/hooks/nobr_emoticons.py (optional)
• Define action interface (inputs): mkdocs_config, docs_dir, site_dir (defaults aligned with whattodo).
• In action: install deps (Python + mkdocs), install/setup pandoc, run the 3-stage pipeline.
• Provide example workflow snippet for content repos.
• Add minimal CI (optional): run a smoke build on a sample fixture repo.

Notes

• Target audience: non-developers. Default experience should be “edit markdown in GitHub UI → commit → site/epub updated”.
• Keep bongiozzo/whattodo as the canonical text + history repo.