Tutorial(s) for new programmers

[byorgey]

The current tutorial is written with the goal of describing the language as quickly as possible to contributors who are already familiar with functional programming. It is not at all appropriate for new programmers (as many of the target audience will be). We should start writing some much gentler tutorials (with exercises, etc.).

[byorgey]

Creating files with comments and code in it, with definitions to be filled in by the user (with accompanying properties) seems to work well.

[byorgey]

The reference documentation helps somewhat. But after this semester is over, I should start extracting exercises from some of the Discrete assignments I have made (+ creating more), with the goal of being able to use the results in future offerings of the course.

[byorgey]

Started working on this in branch docs/introduction.

[byorgey]

See #406.