doc: update top level README.md to bring it up to date and be more useful (#4111)

* doc: update top level README.md to bring it up to date and be more useful as an overview

Signed-off-by: Stewart X Addison <[email protected]>

---------

Signed-off-by: Stewart X Addison <[email protected]>
Co-authored-by: Stewart X Addison <[email protected]>

Author: sxa

.github/ISSUE_TEMPLATE/jenkinsupdate.md

@@ -6,7 +6,7 @@ labels: 'jenkins'
 assignees: ''
 
 ---
-Process to follow: https://github.com/adoptium/infrastructure/blob/master/README.md#jenkins
+Process to follow: https://github.com/adoptium/infrastructure/blob/master/docs/JenkinsMaintenance.md
 
 Current version: X.YYY.Z - (Look in bottom right of the jenkins server
 New version: TBC