Include JIRA Release Notes Description

The automation creating the Release Notes pull request on the documentation website when a release is promoted only includes the "Release Notes Summary" JIRA field.

We want to include also the "Release Notes Description" field, if not empty, after the Release Notes Summary.

The pipeline generating the release notes is https://jenkins.platform.dev.nuxeo.com/job/nuxeo/job/lts/job/draft-release-upgrade-notes-2021/
Related groovy file: https://github.com/nuxeo/integration-scripts-priv/blob/2021/Jenkinsfiles/draft-release-upgrade-notes.groovy

The current method computeContent needs to be updated to retrieve the Release Notes Description field for each issues.
The release note summary should be on a new field alongside "key", "summary", "body", the "body" should then be the release note description.

Update the writeReleaseNoteDocPage method to write the Release Note Summary and then the RN Description if not empty.