Document devpi release flow and stable fallback

Jenkinsfile

@@ -35,6 +35,30 @@ pipeline {
         stage('Build') {
             steps {
                 sh '''
+                    STABLE_INDEX_URL=${STABLE_INDEX_URL:-https://pypi.mandelblog.com/mandel/stable/+simple/}
+                    TESTING_INDEX_URL=${TESTING_INDEX_URL:-https://pypi.mandelblog.com/mandel/testing/+simple/}
+                    ROOT_INDEX_URL=${PIP_EXTRA_INDEX_URL:-https://pypi.mandelblog.com/root/pypi/+simple/}
+                    export STABLE_INDEX_URL
+                    if python3 - <<'PY'
+import os
+import sys
+from urllib.request import Request, urlopen
+from urllib.error import URLError, HTTPError
+url = os.environ["STABLE_INDEX_URL"]
+try:
+    req = Request(url, method='HEAD')
+    with urlopen(req, timeout=10) as response:
+        sys.exit(0 if response.status < 400 else 1)
+except HTTPError as exc:
+    sys.exit(0 if exc.code < 400 else 1)
+except URLError:
+    sys.exit(1)
+PY
+                    then
+                        echo "devpi stable index available, but stable-first install is not enabled yet"
+                    else
+                        echo "devpi stable index not available, using testing as production source"
+                    fi
                     if command -v sudo >/dev/null 2>&1 && sudo -n true >/dev/null 2>&1; then
                         sudo apt-get update -y
                         sudo apt-get install -y python3-venv python3-pip make build-essential libpq-dev \
@@ -51,8 +75,8 @@ pipeline {
                     . .venv/bin/activate
                     pip install coverage
                     pip install --upgrade pip "setuptools==69.5.1" wheel
-                    PIP_INDEX_URL=${PIP_INDEX_URL:-https://pypi.mandelblog.com/mandel/testing/+simple/} \
-                    PIP_EXTRA_INDEX_URL=${PIP_EXTRA_INDEX_URL:-https://pypi.mandelblog.com/root/pypi/+simple/} \
+                    PIP_INDEX_URL="$TESTING_INDEX_URL" \
+                    PIP_EXTRA_INDEX_URL="$ROOT_INDEX_URL" \
                     pip install --no-build-isolation --pre --editable . setuptools wheel --upgrade --upgrade-strategy=eager --use-deprecated=legacy-resolver
                     cp "${JOB_BASE_NAME}/ocyan.json" "${JOB_BASE_NAME}/${JOB_BASE_NAME}.json"
                     pip install ruff vdt.versionplugin.wheel