Auto merge of #32230 - GuillaumeGomez:extend_css, r=alexcrichton

Add --extend-css option to rustdoc

Fixes #32223

r? @brson

diff --cc src/librustc_driver/lib.rs
Simple merge

diff --cc src/librustdoc/html/render.rs
Simple merge

diff --cc src/librustdoc/lib.rs
Simple merge