Move docs to HTTPS

[mhils]

We can do that either by reverse-proxying to readthedocs or by just using a service that does that for us, e.g. Cloudflare.

[jmoujaes]

Looking at https://mitmproxy.org/ it seems to be HTTPS now.

[mhils]

This issue is referring to http://docs.mitmproxy.org/. ReadTheDocs unfortunately doesn't support SSL for custom domains out-of-the-box (https://docs.readthedocs.io/en/latest/alternate_domains.html#cname-ssl).