Add description for plugin manager

Adds missing index.jelly to fix plugin's presentation in plugin manager and plugin site. https://issues.jenkins.io/browse/JENKINS-68300

[x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
[x] Ensure that the pull request title represents the desired changelog entry
[x] Please describe what you did
[x] Link to relevant issues in GitHub or Jira
[n/a] Link to relevant pull requests, esp. upstream and downstream changes
[n/a] Ensure you have provided tests - that demonstrates feature works or fixes the issue

jenkinsci / ghprb-plugin