Document and add examples for each Block type

[bbrowning]

We need to document what each Block type does - new and old ones - as well as add examples for how to use them from a pipeline config. This could be an entire section of our README.md or broken out into a separate document that we link to from README.md.

[asaluja-rh]

@bbrowning whenever we do this, can we extend this to also document num_samples for custom skills, allowing user to tweak skill knobs, making skills more useful to the end user.

[bbrowning]

Providing simple examples of how to use the various block types requires us to add a simple CLI in SDG to run a single data generation pipeline, inspired from the one the research team uses in their work.

A PR for that is at #492 and once that merges we can start adding a lot more examples of Pipelines, Blocks, etc for all the different options we support. That PR includes a way to run every example as an automated test, so we ensure the examples all work and continue to work as we change things.