Docs: Tutorial

[daveyarwood]

See #8 (comment) for context.

The tutorial sections could be something like:

• Anatomy of an Alda Score (a broad overview of the structure of a score file, explaining instrument parts, variable definitions, etc. without going into too much detail)
• Notes and Rests
• Chords and Voices
• Grouping Instruments (covers instrument groups and naming instruments/groups)
• Attributes
• Advanced Rhythms (covers second/millisecond durations and CRAM notation)
• Using Variables
• Generating Music by Writing Code (covers inline Clojure code and an overview of the alda.lisp DSL)

This is just a quick sketch; I'm definitely open to any suggestions about how to tweak this, but maybe this outline can be a good starting point.

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.

[daveyarwood]

NOTE: There is already the "official" tutorial on the Alda website, but I think it would be great to have a revamped tutorial broken down into sections, with a sidebar, with better explanations and more examples. The current tutorial also does not cover some of the more advanced concepts.