Migrate docs from Sphinx to MkDocs (#18145)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/mkdocs/hooks/remove_announcement.py

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: Apache-2.0
+import os
+from typing import Literal
+
+
+def on_startup(command: Literal["build", "gh-deploy", "serve"], dirty: bool):
+    # see https://docs.readthedocs.io/en/stable/reference/environment-variables.html # noqa
+    if os.getenv('READTHEDOCS_VERSION_TYPE') == "tag":
+        # remove the warning banner if the version is a tagged release
+        docs_dir = os.path.dirname(__file__)
+        announcement_path = os.path.join(docs_dir,
+                                         "mkdocs/overrides/main.html")
+        # The file might be removed already if the build is triggered multiple
+        # times (readthedocs build both HTML and PDF versions separately)
+        if os.path.exists(announcement_path):
+            os.remove(announcement_path)