ci: add build-only docs verification job to ci.yml

[wphillipmoore-vergil-agent]

Task

Add the build-only docs verification job to this repo's
.github/workflows/ci.yml, so docs-build failures are caught pre-merge instead
of post-merge in CD.

Change

Add this job to the jobs: map in .github/workflows/ci.yml. The job id must
be exactly docs — branch protection requires the check context docs / docs,
which is derived from this caller-job id.

  docs:
    uses: vergil-project/vergil-actions/.github/workflows/ci-docs.yml@v2.0

No inputs are needed: the reusable workflow defaults to docs/site/mkdocs.yml
and the prod container, and self-guards to a no-op pass if no mkdocs config is
present.

Acceptance

• ci.yml has a docs: job calling ci-docs.yml@v2.0.
• The job appears as check context docs / docs on PRs.
• vrg-validate passes.

Context

Part of the fleet docs-verification rollout. Action code:
vergil-project/vergil-actions#664 / vergil-project/vergil-actions#665.
Required-check enforcement: vergil-project/vergil-tooling#1356.