add CI check for zstd_manual before release

ensures it's properly updated

.github/workflows/release_check.yml

@@ -0,0 +1,31 @@
+name: check_manual
+
+on:
+  push:
+    branches:
+      - release
+  pull_request:
+    branches:
+      - release
+
+permissions: read-all
+
+jobs:
+  verify-manual:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v3
+
+      - name: Save current manual
+        run: mv doc/zstd_manual.html doc/zstd_manual_saved.html
+
+      - name: Generate new manual
+        run: make manual
+
+      - name: Compare manuals
+        run: |
+          if ! cmp -s doc/zstd_manual.html doc/zstd_manual_saved.html; then
+            echo "The manual was not updated before release !"
+            exit 1
+          fi