Remove link to letsencrypt readthedocs (#7757)
Remove link to letsencrypt readthedocs (#7757)

After a brief discussion in Mattermost, I shut down letsencrypt.readthedocs.io. Turns out we were linking to it in our README here so let's remove the broken link.

I didn't update the link to point to one of the readthedocs projects we still have because are main Certbot docs are self-hosted rather than being on readthedocs.
lmcro/letsencrypt,stweil/letsencrypt,letsencrypt/letsencrypt,stweil/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt