Docs: Developer Portal

[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 Alda Developer Portal will be a place for technical details not necessarily relevant to the typical end-user, but useful for developers who want to know how Alda works under the hood.

It should include information about things like:

• Using Alda in a Clojure program
• Technical details about Alda components
• Writing your own Alda components
• ZeroMQ architecture Alda player OSC API