Add docs, styles, quick-start & FAQ details

Enhance index.html and index_vi.html with a full Documentation section and Quick Start guide, expanded FAQ entries (collapsible details), and multiple use-case examples. Add new CSS variables and styles (h3/h4, info/success/warning boxes, doc-section, workflow, settings tables, OCR cards, badges, TOC, etc.) and new UI classes (btn-success, bmc-btn tweaks). Update navigation links and reorder sections; simplify footer and include version/license links. Remove/condense some older sections (legacy settings/shortcuts blocks) and sync Vietnamese page with the same documentation and style updates.

Author: thanhkeke97