Add an `Example` field to the mixlib-cli spec

Motivation

As a technical writer
I want to add examples to the CLI docs
so that users will know the tricky bits and we can automate the CLI docs

The documentation team is working towards meaningful automation of our reference documentation (CLI, API, config). Hand-curating CLI examples and scattering them throughout the documentation by means of dtags, partials, or includes makes the documentation lifecycle difficult to manage. Adding an example field that can handle (or, at the very least, ignore) command line examples such as --bootstrap-vault-json '{ "vault1": ["item1", "item2"], "vault2": "item2" }' would help both users and the docs team.

Specification

:example - An example showing the syntax to use with this option. Example "--bootstrap-vault-json '{ "vault1": ["item1", "item2"], "vault2": "item2" }' "

Downstream Impact

This would impact the formatter and how we output the CLI docs, both on the command line and into .rst or .md

Start by locating the mixlib-cli option spec and the formatter paths mentioned in the issue. Trace how option metadata is emitted on the command line and into .rst or .md output, then define the Example field behavior and verify that command-line examples are handled correctly in each output format.