Docs: Better landing page

[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.

Quoting myself:

I made a quick sketch of what these sections might look like:

From the landing page, sections could include:

• What's new in Alda 1.0.0? (1.1.0, 1.2.0, etc.)
• Setup and Usage
• Tutorial (see below)
• Reference (would include things 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)
• The Alda Command-Line Client
• Developer Portal (would include information like...)
  • Using Alda in a Clojure program
  • Technical details about Alda components
  • Writing your own Alda components
  • ZeroMQ architecture
• Community (links to the Alda Slack group, subreddit, GitHub org, etc. and information about contributing to Alda)

Quoting @abhi18av:

@daveyarwood Let's include the your talk videos in the docs.

Alda: A Music Programming Language, Built in Clojure (David Yarwood)

Alda: A text based music composition language

I think we could also include a "Talks and Demos" section where we could include links to the videos above, as well as this excellent demo by @jimcheetham.

[daveyarwood]

Another great idea from @abhi18av:

Also I feel we need to expand on the examples to something more like a gallery.

I think an "Example Scores" section would be nice. We could adapt the example scores in the alda-core repo and come up with a cohesive set of examples that serve as good demos for Alda's features.

The example scores in the alda-core repo are used for automated testing, so many of them are not really the best demos. We should think about which ones to include and possibly about adding additional examples.