Refactor md-to-HTML; add sections and CLI

Major refactor of the Markdown→HTML converter into modular utilities and clearer dispatching. Adds heading/TOC utilities (extract_headings, generate_toc_html, add_ids_to_headings), markdown transformation helpers (remove_inline_links, demote_headings), section extraction and builders (extract_section, build_section_md), and HTML assembly/writing helpers (build_html, write_file). Introduces per-document processors (process_whats_new, process_user_guide, process_generic), simplifies convert_file dispatcher, and enhances the CLI with --only, --no-toc, --no-shortcuts and --no-marks options. Also tightens external link handling, fixes stylesheet link formatting, and improves docstrings/comments and overall code organization.

Author: QaisAlrefai