Link to specific platform installation instructions

[dra27]

https://ocaml.org/install automatically displays the Windows page when viewed from a Windows machine, but it would be nice if it updated the URL with an anchor (i.e. such that https://ocaml.org/install#windows activates the Windows instructions anywhere and https://ocaml.org/install#{unix,linux,macos,bsd,etc.} activates the Linux instructions) so that it can be referenced directly.