Docs: Host documentation at docs.alda.io

[daveyarwood]

See #8 for context.

I think the Python documentation is a good source of inspiration. There are some nice features here:

• The documentation is available by release version. For example, if you're using Python 3.5 instead of 3.6, you can find the latest 3.5 version (i.e. 3.5.3) docs here.

• The landing page has big links to the most relevant information, like the changelog, a tutorial, installation instructions, an FAQ, etc.

• As you navigate through the docs, there is a sidebar with quick links to each section, making it easy to zero in on a particular section.

I previously looked into http://readthedocs.io, and it seems like a nice way to set up something like this. I did a quick port of our existing documentation here: http://alda.readthedocs.io/en/latest/

The docs are actually generated from the alda-lang/alda repo, which is great because we can still keep the docs version controlled alongside the source code, and easily generate different versions of the documentation based on Alda release versions.

The next steps are to organize the docs better so that they look good on readthedocs.io, and then we could CNAME the docs to something like http://docs.alda.io and include a link on the website.

[daveyarwood]

Note: we should wait to do this until the docs are in a more presentable form (see other issues about improving the documentation).

[daveyarwood]

Assigning myself because I am able to set up the CNAME for docs.alda.io.

[daveyarwood]

The last time I thought about this, I was leaning towards converting the docs into Asciidoc and using Antora to generate a documentation site. But it looks like recently, there are some newer options like Docusaurus that are worth exploring. (If the tool is good enough, I don't really care strongly about whether we maintain the docs in Asciidoc vs. Markdown)