docs: add Vellumli wiki - Home with versions table (26.2 / 1.21.1), getting started, book.json, categories and entries, page types, text formatting, templates, API (per-version signatures), configuration, migrating from Patchouli - Patchouli attribution, CC BY-NC-SA 3.0 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>