Explicitely show broken reference in package documentation

[dbuenzli]

Note: this has been selected as an Outreachy issue. To be assigned, you must have completed the instructions in #1553.

Task

Currently, broken references in the package documentation are not easy to notice. We should add some CSS to highlight them.

Here's a page with such a broken reference ("a quick example").

What you need to do:

Add CSS to https://github.com/ocaml/ocaml.org/blob/main/asset/css/doc.css to style the broken references:

.xref-unresolved {
    ...
}

Broken reference should be styled to look like this:

[oyenuga17]

Hi @sabine Please I'm interested in working on this issue. I have read through the Outreachy Winter 2023 - 2024 doc, and I have successfully ran the hello-world project

[SaySayo]

Have you setup the project according to this guidelines depending on the outreachy project you're applying to? If you have, please send a screenshot.

[oyenuga17]

Have you setup the project according to this guidelines depending on the outreachy project you're applying to? If you have, please send a screenshot.

@SaySayo

[SaySayo]

@oyenuga17 you will be assigned this issue, let me know if you need any pointers. cc @sabine