Add a _selftest.yml guard: fail if a reference page has no website sidebar entry

[d-morrison]

Follow-up from #273 / #429 (the reviewer and the issue both suggested it).

Problem

A new website/reference/<name>.qmd page can land without a matching entry in website/_quarto.yml's Reference sidebar, leaving it reachable only by direct URL. #273 filed with 4 missing pages; by the time #429 fixed it, the set had drifted to 8 -- direct evidence that manual upkeep does not hold.

Proposal

Add a step to _selftest.yml (or a small script it runs) that fails when any website/reference/*.qmd is absent from website/_quarto.yml's sidebar:

missing=0
for f in website/reference/*.qmd; do
  b=$(basename "$f")
  grep -q "reference/$b" website/_quarto.yml || { echo "::error::$b missing from _quarto.yml sidebar"; missing=1; }
done
exit $missing

Optionally also assert the sidebar order matches website/workflows.qmd's catalog order (as #429 verified by hand), so the two stay in sync. This is the algorithmatize-checks move: turn the manual grep #273/#429 ran into a standing check.