fix: language manual redirect, remove unnecessary append of index.html

[sabine]

Related: #2469

Bug:

https://ocaml.org/manual/attributes.html

redirects to

https://ocaml.org/manual/5.2/attributes.html/index.html

This patch should fix this.

[cuihtlauac]

Some 404 remains, e.g.:

http://localhost:8080/manual/api/compilerlibref http://localhost:8080/manual/latest/

This branch fixes them: https://github.com/ocaml/ocaml.org/tree/fix-more-redirect (and makes the code more readable)