Stdlib documentation is not available for 4.11 and earlier

[EmileTrotignon]

This is dead: https://ocaml.org/manual/4.11/api/index.html
and preceding versions too.

[sabine]

This could be fixed by placing a stdlib API docs under https://github.com/ocaml-web/html-compiler-manuals/tree/main/4.11. Need to build the compiler docs from source in https://github.com/ocaml/ocaml