feat(doc): surface the docs preview artifact in the PR job summary

[Minipada]

Summary

doc.yaml (#252) already uploaded the built mdbook site as a workflow artifact (doc-site) on every run, including PRs — nothing restricted that step to push. What #135 actually needed was discoverability: the artifact was buried in the run's Artifacts section, several clicks past where a reviewer lands from the PR's Checks tab.

• Give the Upload the built site step an id and, on pull_request runs, read its artifact-url output (available immediately post-upload since upload-artifact v4) into an env var and write a Markdown download link to $GITHUB_STEP_SUMMARY — the same page "Details" on a PR check opens.
• Stayed artifact-only rather than a hosted deploy: this repo has no preview host configured (Preview documentation in each PR (netlify?) #184 closed without one), and reaching fork PRs with a hosted preview would need pull_request_target plus a deploy token — secrets exposed to untrusted PR code. pull_request keeps this safe; its default GITHUB_TOKEN is read-only, which is all an artifact upload needs, so fork PRs get the same preview link as same-repo PRs.
• Updated the workflow's header comment to document the new behavior and the reasoning for staying artifact-only.

Acceptance criteria (from #135)

• Preview (artifact download at minimum) reachable from the PR checks — one-click link in the job summary
• Works for fork PRs, or degrades gracefully with the limitation documented — pull_request trigger, no secrets, documented in the workflow header
• No secrets exposed to untrusted PR code — no secrets used at all

Test plan

• pre-commit run --files .github/workflows/doc.yaml — check-yaml passes; the build-doc hook (real strictdoc export + mdbook build) passes unaffected
• Not verified end-to-end against a live Actions run (would need this PR's own CI to actually execute the new step) — artifact-url output and $GITHUB_STEP_SUMMARY are documented, stable GitHub Actions features

Closes #135

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

https://claude.ai/code/session_01BxvjR5XyyaEPBBpwGfUK9W

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.50%. Comparing base (1fd33b6) to head (81c41ce).

Additional details and impacted files

@@           Coverage Diff           @@
##            jazzy     #314   +/-   ##
=======================================
  Coverage   40.50%   40.50%           
=======================================
  Files          82       82           
  Lines        5094     5094           
=======================================
  Hits         2063     2063           
  Misses       3031     3031           

Flag	Coverage Δ
cpp-jazzy	40.50% <ø> (ø)

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