Document Spotless

[binkley]

See #462 for the epic.
The card is marked MVP as it is in progress. Remove the "mvp" label if that makes sense.

Context

Show Spotless in the Gradle and Maven build even though Checkstyle remains the example default.
So both Gradle & Maven have flags/settings that do not run Spotless, but you can still run them specifically on the command line.

Tasks for this story

• 1. Configure for Gradle and Maven to format the same as Checkstyle (but Spotless still disabled as a default) .
• 2. Document both Checkstyle and Spotless as options in Gradle and Maven; the project should keep defaulting to Checkstyle. Do not promote a particular style. <-- should we switch to Spotless? We've chatted before on this, and I am still on the fence
• 3. If feasible, show how to set up alternative styles (like how Checkstyle does with files). With Gradle & Maven, it is one-liners to change the style.
• 4. Writing layout and style should be common among the quality plugins. This is a good opportunity to set a common writing pattern so other plugins can follow Spotless/Checkstyle as a template.

I removed the first task: it is unreasonable for a book to deal with slight formatting changes; developers using the writing can figure that out. 😄

Acceptance criteria

Scenario: Reading the book
GIVEN the documentation in the book
WHEN a reader reads about code style checks
THEN there is an example in the book and example code for configuring Spotless to check style for both Gradle and Maven