Add extra docs check.

tests/sanity/extra/extra-docs.json

@@ -0,0 +1,10 @@
+{
+    "include_symlinks": false,
+    "prefixes": [
+        "docs/docsite/"
+    ],
+    "output": "path-line-column-message",
+    "requirements": [
+        "antsibull"
+    ]
+}

tests/sanity/extra/extra-docs.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+# Copyright (c) Ansible Project
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+"""Check extra collection docs with antsibull-lint."""
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+import os
+import sys
+import subprocess
+
+
+def main():
+    """Main entry point."""
+    if not os.path.isdir(os.path.join('docs', 'docsite')):
+        return
+    subprocess.run(['antsibull-lint', 'collection-docs', '.'])
+
+
+if __name__ == '__main__':
+    main()