Use HTTPS links in documentation and comments

It appears there are quite a few `http://` links in the documentation and code comments of this project despite the target websites supporting HTTPS (sometimes HTTPS has to be explicitly requested because the website does not upgrade connections on its own).

A quick search for `http:` yielded about 4000 findings, however I assume a lot are false positives (e.g. because they are part of an XML namespace declaration) and some target sites might not support HTTPS, or are not properly configured (outdated certificate, wrong domain name).

However, for the websites which do support HTTPS it would be good to update the links. Otherwise for `http://` links the browser will first send an unencrypted HTTP request which an attacker in the same network is able to see and for which they can spoof a response.

Slightly related: #4379