hack: add docs generation/validation

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

hack/validate-docs

@@ -0,0 +1,29 @@
+#!/usr/bin/env sh
+set -eu
+
+case ${1:-} in
+  '')
+    . $(dirname $0)/util
+    buildxCmd build \
+      --target validate \
+      --file ./hack/dockerfiles/docs.Dockerfile \
+      .
+    ;;
+  check)
+    status="$(git status --porcelain -- docs/reference 2>/dev/null)"
+    diffs=$(echo "$status" | grep -v '^[RAD] ' || true)
+    if [ "$diffs" ]; then
+      {
+        set +x
+        echo 'The result of ./hack/update-docs differs'
+        echo
+        echo "$diffs"
+        echo
+        echo 'Please vendor your package with ./hack/update-docs'
+        echo
+      } >&2
+      exit 1
+    fi
+    echo 'Congratulations! All docs changes are done the right way.'
+    ;;
+esac