Add cheatsheet to the odoc manual

[panglesd]

This adds a cheatsheet to the odoc manual. Have a look at the rendered page here.

The cheatsheet is rendered by odoc, which is not really focused on rendering cheatsheets, so it does not render ideally well. If someone wants to contribute by adding some special css on this page to make it better, feel free!

[dbuenzli]

It's great ! I would actually put a direct link at the top here before the first title.

[panglesd]

When this is merged, I think it would make sense to backport it to 2.4 branch and update the website.

[dbuenzli]

@panglesd thanks for this excellent documentation addition (note that this could not have been achieved[^1] with ocamlmark ;-)

[^1]: Except by dropping to HTML and thus leaving other backends in the dust.