Docs: What's new in Alda X.X.X?

[daveyarwood]

Part of #8.

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 way readthedocs.io is set up, documentation versions are automatically mapped to branches of the repo. The latest version in each series of Alda releases (e.g. 1.0.X, 1.1.X, 1.2.X...) will have its own version of the documentation, corresponding to the docs folder in that branch of the alda-lang/alda repo.

I like the way the Python documentation site handles this:

https://docs.python.org/3 (landing page)
https://docs.python.org/3/whatsnew/3.6.html (summary of changes since the last minor version, 3.5)
https://docs.python.org/3.6/whatsnew/changelog.html (detailed changelog for this series of releases)