moved latest warning banner logic to conf.py (#5508)

Author: tkknight

docs/src/_templates/layout.html

@@ -331,6 +331,16 @@ def _dotv(version):
     },
 }
 
+# if we are building via Read The Docs and it is the latest (not stable)
+if on_rtd and rtd_version == "latest":
+    html_theme_options[
+        "announcement"
+    ] = f"""
+        You are viewing the <b>latest</b> unreleased documentation
+        <strong>{version}</strong>. You can switch to a
+        <a href="https://scitools-iris.readthedocs.io/en/stable/">stable</a>
+        version."""
+
 rev_parse = run(["git", "rev-parse", "--short", "HEAD"], capture_output=True)
 commit_sha = rev_parse.stdout.decode().strip()