Improve old API doc link redirects

[za3k]

Old links such as https://v3.ocaml.org/p/base/v0.15.0 301 redirect to https://ocaml.org, effectively breaking a wide variety of links to ocaml.org across the web.

First, please don't use 301 redirects, in generality, ever. It means that this is now broken for anyone that has followed that link, and there is literally nothing you can do to fix it for those users.

Instead, it should be a 302 redirect to https://ocaml.org/p/base/v0.15.0

(x-ref) janestreet/base#176