ci(docs): treat non-404 4xx link-check failures as warnings, not errors

markdown-link-check has no concept of "warning" vs "error" — it's a
binary alive/dead per link, so a transient 429 from a rate-limiting site
we don't control (recently: blogspot.com, izndgroup.com) fails the whole
CI job exactly like a genuine dead link, with no way to tell them apart
from the job's exit code.

New scripts/check-doc-links.sh wraps the tool per file, parses its
"[✖] <url> → Status: <code>" output, and re-decides pass/fail per link:
404 still fails the build (a real dead link, worth fixing), other 4xx
(429, 401, 403, ...) become a GitHub Actions ::warning:: annotation
instead, and anything else (5xx, timeouts, DNS failures) still fails the
build same as before. De-dupes markdown-link-check's own doubled -v
output. Written for bash 3.2 (macOS's default /bin/bash) so it's testable
locally, not just on the ubuntu-latest runner.

Existing retry config in .github/markdown-link-check.json (retryOn429,
3 retries, 30s backoff) is untouched; this only changes what happens once
retries are exhausted.

Verified locally: 6 synthetic scenarios (404/429/500/mixed/clean/
duplicate-line) via a stubbed markdown-link-check, plus a real run
against the docs tree with the actual tool.

.github/workflows/ci.yml

@@ -197,7 +197,7 @@ jobs:
       - name: Check documentation links
         run: |
           npm install -g markdown-link-check
-          find . -name "*.md" -not -path "./tests/*" -not -path "./node_modules/*" -print0 | xargs -0 -n1 markdown-link-check -q -v -c .github/markdown-link-check.json
+          ./scripts/check-doc-links.sh
 
       - name: Warn on pending images
         run: |