Docs: Setup and Usage

[daveyarwood]

See #8 for context.

This should be done by modifying the docs folder in alda-lang/alda. The docs are kept in sync with releases and used to generate http://alda.readthedocs.io, which will be CNAMEd to docs.alda.io.

The docs should have a page guiding the reader through:

• Installing Alda
• Learning about MIDI soundfonts and optionally installing one to make Alda sound better
• Learning about the available text editor plugins

This information is currently all in the README:

https://github.com/alda-lang/alda#installation
https://github.com/alda-lang/alda#midi-soundfonts
https://github.com/alda-lang/alda#editor-plugins

Once we've moved it to the docs, we can replace those parts of the README with a link to the docs.

I think this page could also include some very basic usage examples like what we have here, and then suggest that the reader either read the Alda Command-Line Client docs for more technical information, or proceed to the Tutorial section for a guided tour of Alda.

[daveyarwood]

Nowadays, the official installation instructions on the website are way better than back in 2017 when I created this issue. I'm happy enough with those instructions that I think we could move them as-is into the new docs site.