Add markdown-formatter plugin to ci.jenkins.io

[MarkEWaite]

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Service(s)

ci.jenkins.io

Summary

Allow markdown formatted job descriptions, comments, and Pipeline documentation on ci.jenkins.io so that we can include hyperlinks, lists, and more in job descriptions without inserting HTML markup

Reproduction steps

1. Open the Pipeline syntax global variables page for our Pipeline library "buildDockerAndPublishImage" and see that the author of the description assumed that markdown is supported
2. The hyperlinks and list on the root page of ci.jenkins.io are a little easier to manage in markdown than in HTML

We can allow markdown formatted descriptions by installing the markdown formatter plugin on ci.jenkins.io

Markdown support for the Pipeline case will need an upgraded Pipeline Groovy library plugin that includes the pull request from @mawinter69:

• https://github.com/jenkinsci/pipeline-groovy-lib-plugin/pull/117

Before a new plugin like the markdown formatter is deployed on ci.jenkins.io, we've liked to have the Jenkins security team review the plugin to assure it is acceptable for a large scale installation like ci.jenkins.io. We need the core-security-review team to review the plugin for security issues