Add OpenShift "No 'Optional:' in header" rule

.vale/fixtures/OpenShiftAsciiDoc/NoOptionalInHeader/.vale.ini

@@ -0,0 +1,5 @@
+; Vale configuration file to test the `NoOptionalInHeader` rule
+StylesPath = ../../../styles
+MinAlertLevel = error
+[*.adoc]
+OpenShiftAsciiDoc.NoOptionalInHeader = YES

.vale/fixtures/OpenShiftAsciiDoc/NoOptionalInHeader/testinvalid.adoc

@@ -0,0 +1,9 @@
+//vale-fixture
+:_mod-docs-content-type: REFERENCE
+[id="optional-postinstall-configurations_{context}"]
+= Optional: Postinstall configurations
+
+== Optional: More postinstall configurations
+
+\\ == Optional: more options
+

.vale/fixtures/OpenShiftAsciiDoc/NoOptionalInHeader/testvalid.adoc

@@ -0,0 +1,4 @@
+//vale-fixture
+:_mod-docs-content-type: REFERENCE
+[id="cli-basic-commands_{context}"]
+= No optional CLI commands

.vale/styles/OpenShiftAsciiDoc/NoOptionalInHeader.yml

@@ -0,0 +1,8 @@
+---
+extends: existence
+scope: raw
+level: error
+link: https://github.com/openshift/openshift-docs/blob/main/contributing_to_docs/doc_guidelines.adoc#assembly-file-metadata
+message: "Do not begin a module or assembly header with 'Optional:'."
+raw:
+  - '(?<!\/\/)={1,5} Optional:'