doc: use Customizable Header for system messages (#4213)

Update the security releases documentation to switch to using the
Customizable Header plugin for system messages. This will show
the message on all pages, including the error page shown to anyone
whose access to Jenkins is restricted during the security release
preparation.

Fixes: #4203

Author: richardlau

doc/jenkins-guide.md

@@ -98,13 +98,21 @@ referencing the discussion post.
 You must be a member of the `nodejs/jenkins-admins` team to have the necessary
 permissions to lock down the CI.
 
-Add a Jenkins "system message" in https://ci.nodejs.org/configure. Something like:
+Add a system message to Jenkins by going to https://ci.nodejs.org/manage/appearance
+and under "Customizable Header" click the "+ Add System message" button. In the
+"Message" field that is created, enter something like:
+
 ```html
 <h1 style="color:red">system is under embargo for a security release</h1>
 <h2>For solidarity, even if you have access, please don't start unrelated jobs</h2>
 <p>Refs: <a href="https://github.com/nodejs/build/issues/xxx">https://github.com/nodejs/build/issues/xxx</a></p>
 ```
 
+Unselect "Dismissible" and change the "Background of System Message" to
+"Warning".
+
+![](../static-assets/jenkins-set-system-message.png)
+
 To make it more obvious that the CI has been locked down, add some "Extra CSS"
 to https://ci.nodejs.org/manage/appearance.
 This should already exist but commented out like this:
@@ -174,8 +182,7 @@ Relevant logs:
 1. system (includes security matrix) - https://ci.nodejs.org/jobConfigHistory/history?name=config
 2. CSS - https://ci.nodejs.org/jobConfigHistory/history?name=org.codefirst.SimpleThemeDecorator
 
-Undo the "system message" changes from https://ci.nodejs.org/configure and
-"Extra CSS" theme changes from https://ci.nodejs.org/manage/appearance.
+Undo the "system message" changes and "Extra CSS" theme changes from https://ci.nodejs.org/manage/appearance.
 
 ## Solving problems