Prevent leaking trackable pageview stats to CDNs by default

This is a kinda continuation of my previous comment here: https://github.com/JuliaDocs/Documenter.jl/issues/2159#issuecomment-1879633205 from #2159 .

It would be great if the generated HTML didn't send referrer URLs in headers, thus preventing the CDNs the default access to (very easily trackable) user page load stats by default. In turn, this would allow much better compliance with various privacy protection guidelines, incl. GDPR.

For the beginning, I think that just pasting a referrerpolicy="no-referrer" into all <link> tags that point to external CDNs would be more than sufficient. See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link#referrerpolicy for details there.

Thanks!