test: add lint rule for heading start

Heading markup should start on the first column of a line, otherwise we
risk it being rendered as a paragraph, per the markdown spec.

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

.markdownlint.json

@@ -1,6 +1,7 @@
 {
   "default": false,
   "hr-style": true,
+  "heading-start-left": true,
   "no-missing-space-atx": true,
   "no-multiple-space-atx": true,
   "no-missing-space-closed-atx": true,

content/security/for-admins/single-sign-on/manage/_index.md

@@ -45,7 +45,7 @@ aliases:
 {{< /tab >}}
 {{< /tabs >}}
 
- ## What's next?
+## What's next?
 
 - [Set up SCIM](../../scim.md)
 - [Enable Group mapping](../../group-mapping.md)