Self-host docs for other Certbot packages

[bmw]

Currently Certbot's API documentation is hosted here, however, our documentation for our other packages like acme and the Apache and Nginx plugins are not. In fact, any references to code in acme goes to readthedocs as you can see here. Can we self host this?

cc @vbrown608

[mfb]

This should be doable by modifying the gulp build script (and adding additional submodules, as we currently use a submodule to import the certbot codebase for building the docs). Someone will need to figure out the desired URL hierarchy for hosting docs from multiple packages.

[bmw]

No rush on this as it's my fault for not bumping this issue sooner, but we need to add documentation about Certbot's DNS plugins in preparation for Let's Encrypt's new server that allows users to obtain wildcard certificates only if they do domain validation over DNS using something like Certbot's DNS plugins. For now, I'm just setting documentation for these packages up on Read the Docs, but it'd be nice if they could be hosted here instead (especially since RTD shows ads now?).

This should probably be a separate issue, but bonus points if we're able to set up some of the automation that RTD has around things like automatically building the documentation when it changes. Perhaps we'd be able to reuse their code? Being able to host multiple versions of the documentation might also be useful, however, I'm not sure we'd use that feature in the long term.

[bmw]

We actually may be able to do this on our end with Sphinx, but we probably should create a meta docs package. Making the documentation for Certbot rely on specific plugins being installed is a bad idea as we ship the documentation as do distros like Debian.