Wrap mainline mode deprecation in alert

Author: asbjornu

docs/input/docs/reference/modes/mainline.md

@@ -7,7 +7,10 @@ Description: |
     to `main`.
 RedirectFrom: docs/reference/versioning-modes/mainline-development
 ---
-_This was deprecated in 6.x.x : see this PR https://github.com/GitTools/GitVersion/pull/3844 _
+
+:::{.alert .alert-danger}
+ **`Mainline` mode was deprecated in version 6 of GitVersion.** See [the following pull request](https://github.com/GitTools/GitVersion/pull/3844) for details.
+:::
 
 Mainline Development is enabled when using [GitHubFlow][githubflow] or any other
 strategy where you develop on `main`. The main rule of mainline development is