Preparatory steps to have https://gdal.org default to a 'stable' branch

.github/workflows/auto_tag_stable.yml

@@ -0,0 +1,47 @@
+name: Update Stable Tag
+
+on:
+  push:
+    branches:
+      - '**'  # Matches all branches, but we later filter on the one matching the STABLE_BRANCH repository variable
+
+permissions:
+    contents: read
+
+jobs:
+  update-stable-tag:
+    runs-on: ubuntu-latest
+    if: github.repository == 'OSGeo/GDAL'
+    permissions:
+        contents: write
+    steps:
+      - name: Check branch match
+        id: check_branch
+        env:
+          STABLE_BRANCH: ${{ vars.STABLE_BRANCH }} # Repository variable
+        run: |
+          echo "Push detected on branch $GITHUB_REF"
+          if [[ "${GITHUB_REF#refs/heads/}" != "${STABLE_BRANCH}" ]]; then
+            echo "This workflow only runs for branch $STABLE_BRANCH. Skipping further steps."
+            echo "run=false" >> $GITHUB_OUTPUT
+          else
+            echo "run=true" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Checkout code
+        if: steps.check_branch.outputs.run == 'true'
+        uses: actions/checkout@v4
+
+      - name: Tag
+        if: steps.check_branch.outputs.run == 'true'
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          git config --global user.email "GDAL-bot@example.com"
+          git config --global user.name "GDAL-bot"
+          touch .dummy-file
+          git add .dummy-file
+          # Do that so that stable doesn't have the same git sha as the stable branch, so ReadTheDocs triggers a build
+          git commit -a -m "Add .dummy-file"
+          git checkout -b stable
+          git push -f origin stable

HOWTO-RELEASE

@@ -152,6 +152,11 @@ Process :
 
 15) Once the vote has positively completed,
 
+15.0) For a new feature release, update the STABLE_BRANCH repository variable
+
+    Go to https://github.com/OSGeo/GDAL/settings/variables/actions/STABLE_BRANCH
+    and update the value to be release/x.y
+
 15.1) Tag the release as final:
 
     git checkout v3.1.0RC1

doc/source/conf.py

@@ -160,7 +160,9 @@ html_theme_options = {
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ["_static"]
-html_extra_path = ["../build/html_extra"]
+
+# For generated content and robots.txt
+html_extra_path = ["../build/html_extra", "extra_path"]
 
 html_js_files = ["announcement.js"]
 html_css_files = ["announcement.css"]

doc/source/extra_path/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /en/stable/
+Disallow: /en/
+Sitemap: https://gdal.org/sitemap.xml