Documentation provides invalid SSL certificate

Currently, https://llvmlite.pydata.org/ serves an invalid certificate (valid only for readthedocs.org), but SSL on custom domains in Read the Docs should work more or less out-of-the-box. According to [the documentation](https://docs.readthedocs.io/en/stable/custom_domains.html#custom-domain-ssl):

> By default, when you setup a custom domain to host documentation at Read the Docs, we will attempt to provision a domain validated SSL certificate for the domain. This service is generously provided by Cloudflare.

From that same page, it would seem that one needs to use readthedocs.io (note the io instead of org) in the CNAME record to make use of this functionality though:

> Some older setups configured a CNAME record pointing to readthedocs.org or another variation. While these continue to resolve, they do not yet allow us to acquire SSL certificates for those domains.

And this is probably the reason it fails for llvmlite:

```
$ dig llvmlite.pydata.org | grep CNAME
llvmlite.pydata.org. 136 IN CNAME llvmlite.readthedocs.org.
```

So presumably, if this functionality is something you would want to make use of (which would be nifty), it seems that all it takes is an update to the CNAME record and a bit of clicking around in the Read the Docs control panel:

> Point the CNAME to readthedocs.io, with no subdomain, and re-request a certificate by saving the domain in the project admin

Start by checking the llvmlite.pydata.org CNAME and the custom-domain settings in the Read the Docs project admin. Point the CNAME to readthedocs.io, re-request the certificate by saving the domain, and verify that the site serves a valid certificate.

Written by the indexing model from the issue text.