Update windows install instructions to opam

[Sudha247]

Fixes #2654.

cc @dra27 @klakplok @pirbo

[cuihtlauac]

@jonahbeckford: Can you review this please?

[jonahbeckford]

Overall it looks good.

When I tried to install ocamlformat with opam 2.2 (unrelated to this PR), it failed. So it would be good to go through the site and see what works and what doesn't.

[jonahbeckford]

Note: I'll submit a DkML PR for ocaml.org early next week. But it is best to get opam 2.2 properly documented on the website first.

[Sudha247]

When I tried to install ocamlformat with opam 2.2 (unrelated to this PR), it failed. So it would be good to go through the site and see what works and what doesn't.

Indeed, I've been making some notes on the state of platform tools on Windows. I'll publish it on gist next week and find a more permanent place for it.

[pirbo]

Thanks @Sudha247. I'll duplicate my remark that I think we should move fast on this at the price of postponing instruction about the platform which is not there yet (and make our priority number 1 to fix that!)

[dra27]

Looking good, thank you! I agree with @pirbo that it would be better to get a version deployed quickly and then iterate on it - to that end, I'd suggest leaving the updates for 3_01_ocaml_on_windows.md to a subsequent PR.

[jonahbeckford]

Edit: I'll do a PR to add back in DkML as a secondary option in mid-Sept. I'm +1 for getting this PR merged.

[christinerose]

Looks like my previous commit to remove the repeated word was lost somehow, so I made a suggestion to do so this time instead of editing it directly.

[dra27]

Is this able to be merged, please - it would be great to have the site up-to-date before tomorrow's ICFP workshop talks!