Add noxfile and workflow to test docsite PRs (#1794)

* add noxfile with mkdocs build session

* add nox build check

* include reusable nox in ci workflow

noxfile.py

@@ -0,0 +1,20 @@
+import nox
+
+
+@nox.session
+def build(session: nox.Session):
+    """
+    Build the AWX Operator docsite.
+    """
+    session.install(
+        "-r",
+        "docs/requirements.in",
+        "-c",
+        "docs/requirements.txt",
+    )
+    session.run(
+        "mkdocs",
+        "build",
+        "--strict",
+        *session.posargs,
+    )