• documentclass: book tells pandoc that the first-level headers should be treated as chapters instead of sections. If you change documentclass, you may need to specify an additional pandoc argument, --chapters
• To disable MathJax, you need to set math: false in index.Rmd.
• bookdown::gitbook() storage configurations in \$post_processor(), only this function can assess the gh_config object.
• When your .Rmd contains Chinese, you need to call rmarkdown::render(encoding = 'UTF-8'), but you mustn’t set options(encoding = 'UTF-8').