doc-sync: add support for uploading the documentation for main

It will refuse running on any other branch than main or stable branches.

Also update the release instructions to run it on the stable branch.

tools/sync-docs.py

@@ -95,19 +95,27 @@ def main(version, directory, www_target):
     index_filename = os.path.join(directory, "index.json")
     nav_filename = os.path.join(directory, "nav.js")
 
+    current_branch = subprocess.check_output(["git", "branch", "--show-current"], text=True).strip()
+
+    if current_branch != 'main' and not current_branch.endswith("-stable"):
+        sys.exit("doc-sync should only be run from main or a stable branch")
+
     for filename in glob.glob(os.path.join(directory, "*.html")):
         process_file(filename)
 
+    if current_branch == "main":
+        version = "devel"
+        dirs = ["devel"]
+    elif int(version) == get_latest_version():
+        dirs = [version, "latest"]
+    else:
+        dirs = [version]
+
     with open(nav_filename, "w") as f:
         f.write(NAV_JS)
 
     update_index_file(version, index_filename)
 
-    dirs = [version]
-
-    if int(version) == get_latest_version():
-        dirs.append("latest")
-
     for d in dirs:
         subprocess.check_call(
             [