Docs: Reference section

[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 Reference section would include information like:

• list of instruments
• list of attributes
• alda.lisp API docs (i.e. a list of functions available in the alda.lisp namespace, what they do and how to use them)