Incorporate documentation command testing

Use the Sourcerer module (currently in ReleaseHx but to be released as its own gem) to test commands in the docs.

In .adoc files, add `[.testable]` role to literalblocks that are meant to be run exactly as displayed, in an environment that follows from the sequence of commands before it, assuming `bundle install` up front.

Then add to Rakefile a task `:readme_test` like those in [releasehx](/DocOps/releasehx/blob/main/Rakefile) and [releasehx-demo](/DocOps/releasehx-demo/blob/main/Rakefile) (neither yet updated as of time of this posting).