Add reference to the Standalone DNS Authenticator

[siilike]

Updated documentation to add a reference to the Standalone DNS Authenticator, https://github.com/siilike/certbot-dns-standalone

[bmw]

Thanks for the PR @siilike.

This change seems to break the table after activating Certbot's development virtual environment and building the docs. If you fix this, I'll happily merge the PR.

We update it regularly regardless, but if you'd like this change to go live ASAP after it lands, you can submit a PR to update the _docs submodule at https://github.com/certbot/website which I'd also be happy to review.

Also to give you a bit of an update on #6646, we're a little swamped with other stuff and will likely to continue to be for months but do plan to review that PR when we can. I'm very sorry it has taken so long.

[siilike]

Thank you for your reply and sorry for not testing the change. Now the table is fixed.

Regarding #6646, it's easy enough to install and use it as a plugin, so don't worry about that, although would be great if it eventually will get merged.

[siilike]

Thank you.