docs: rewrite the readme and add a contributing guide

[tas50]

The README was a mix of usage docs and a generic fork-and-branch section, with
links still pointing at pre-transfer repositories and master. It is now just
what a user of the plugin needs: what it does, what it requires, where tests go,
and what a passing test looks like. Everything a contributor needs moved to a
new CONTRIBUTING.md, modelled on the one in test-kitchen/busser -- how to set the
project up, how to run the suite, the linters CI runs, and the Conventional
Commits convention release-please reads.

Every development dependency now carries a version floor at its current major,
so a resolver cannot quietly pick an ancient release: cookstyle 9, aruba 2.4,
cucumber 11.1, rake 13.4, simplecov 1.1.

Test tooling brought up to date:

• .rubocop.yml targeted Ruby 3.1 while the gemspec requires 3.2.
• The After hook setting Cucumber.wants_to_quit is replaced by cucumber's
  own --fail-fast, which is the supported way to stop at the first failure.
• -x (--expand) is dropped; it only affects Scenario Outline output and
  there are no outlines here.

.simplecov is removed: SimpleCov auto-loads it on require and started a "gem"
profile there, while env.rb separately set a command name, so which profile
applied depended on load order. The whole thing is now in env.rb behind
COVERAGE.

The dead @aruba_timeout_seconds Before hook is also replaced here. aruba 2
removed it, so it was a no-op and commands ran on aruba's 15 second default;
the sibling plugins already got this fix in their modernization pull requests.