[3.14] gh-141811: Split up `init.rst` into multiple pages (GH-144844) (GH-145061)

(cherry picked from commit 60f3c396fe5dc56bc3a56341e2d31fd6061bb068)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Benedikt Johannes <benedikt.johannes.hofer@gmail.com>