Add a check to prevent insecure links in the documentation

[sarayourfriend]

Current Situation

https://github.com/WordPress/openverse/issues/3266

It's possible to add insecure links to the documentation and easy to miss.

Suggested Improvement

We can add a check of some sort (linter or custom Sphinx extension or, if a Sphinx extension exists, a pre-built one, though I couldn't find one) to alert if we accidentally add an insecure link to the documentation.

We can also add a small CSS rule to highlight these cases:

a[href*="http://"] {
  border: 3px solid red;
}

The CSS adds a red border around insecure direct links. It intentionally ignores relative links under the assumption the protocol should already indicate a secure connection.

Benefit

Prevent accidentally adding insecure links.