Docs: show `ctrlrun verify` in a CI that is not GitHub Actions

[arpanghoshal]

ctrlrun verify runs the kernel's own failure scenarios against your configuration and reports
pass, fail or not applicable per guarantee. There is a GitHub Action for it
(action.yml) and a guide that assumes it
(docs/guides/verify-in-ci.mdx).

Nothing in the docs shows it on GitLab CI, CircleCI, Jenkins, Buildkite or Azure Pipelines —
grep -ri gitlab docs/ returns nothing. For anyone not on GitHub the answer is currently "read
the exit codes and work it out."

Pick one CI system you actually use and add the snippet.

Done means

• A section in docs/guides/verify-in-ci.mdx, or a sibling page linked from it, with a working
  pipeline definition for that system.
• It keys off the documented exit codes rather than parsing output —
  docs/reference/exit-codes.mdx.
• If it renders a badge or a summary, the wording is the one in the docs already: the badge
  means declared guarantees pass, never "this agent is secure", and N/M counts
  applicable guarantees. Not applicable is not a pass and there is no flag that folds one
  into the count.
• It carries the same honesty as the existing guide's "What verify cannot see" section, or
  links to it.
• docs/STYLE.md's four checks pass, and the docs audit's link and anchor check resolves.

Say which system you're taking in a comment. More than one person can work on this issue as
long as they take different systems.