Do extra docs validation; explicitly disallow semantic markup in docs (#6345)

* Do extra docs validation. Explicitly disallow semantic markup in docs.

* Forgot to add new requirement.

* Fix prefixes.

* Remove superfluous condition.

* TEMP - make CI fail.

* Revert "TEMP - make CI fail."

This reverts commit 14f4d6b503.

* Remove unnecessary import.

* Make sure ANSIBLE_COLLECTIONS_PATH is set.

* Make sure sanity tests from older Ansible versions don't complain.

tests/sanity/extra/extra-docs.json

@@ -1,10 +1,13 @@
 {
     "include_symlinks": false,
     "prefixes": [
-        "docs/docsite/"
+        "docs/docsite/",
+        "plugins/",
+        "roles/"
     ],
     "output": "path-line-column-message",
     "requirements": [
+        "ansible-core",
         "antsibull-docs"
     ]
 }