New features: added txt and md output formats.
- Added semantic Markdown rendering with headings, links, tables, fenced code blocks, stable anchors, linked TOC, and linked endnotes.
- Added readable plain-text rendering with headings, aligned tables, image placeholders, and footnote support.
- Added document.images.markdown config: placeholder, external, embedded.
- Markdown external images are written to a book-id-specific asset directory.
- Markdown embedded images use data URIs; SVGs are rasterized before embedding.
- TXT always uses image placeholders.
- TXT/Markdown support existing footnote modes: default, float, and floatRenumbered.
- Vignettes are ignored for TXT/Markdown output.
- Updated guide and README documentation.
Full Changelog: v1.4.5...v1.5.0