vignette to showcase an example analysis using each supported class

[corybrunson]

It would probably be useful for users to be able to skim a single vignette containing of short data analyses using ordination techniques supported by ordr. These should not be a mere biplot gallery but rather a set of self-contained studies, separated into sections, each with a clear and concise question or hypothesis, an explanation of the data, several exploratory and/or explanatory analysis steps, and a conclusion.

The vignette could be prepared gradually, one section at a time. Ideally these would not duplicate the examples, and in particular different data sets would be used, though with minimal additional package dependencies.