Add PRdoc check (#1408)

* Add test prdoc
* Prepare for the check
* Escape PR number
* Fix conditional step
* Add checkout and actual check
* Cleanup
* Minor fixes
* Add doumentation
* Add more doc

prdoc/pr_1408_prodc-introduction.prdoc

@@ -0,0 +1,19 @@
+# This PR does not need a prdoc but it is provided in order to test
+title: PRdoc check
+
+doc:
+  - audience: Core Dev
+    description: |
+      This PRdoc is an **example**.
+
+      This PR brings support and automated checks for documentation in the form of a
+      [`prdoc`](https://github.com/paritytech/prdoc/) file.
+
+migrations:
+  db: []
+
+  runtime: []
+
+crates: []
+
+host_functions: []