update docs for new contributors

Suggested changes to contributor-onboarding:

• added documentation/Ironbank_members directory
  • added Intro_to_Ironbank.md: a high level overview of the Ironbank workflow & processes
  • added Hardening.md: an example guide for hardening a container application
  • added Justifications.md: an example guide for justifying vulnerabilities
  • added examples directory
    • added example_issue.md: an example of a completed issue
    • added active-mq-artemis_2.17.0-169205-justifications.xlsx: an example of a completed justification spreadsheet
• updated documentation/Whitelist_Procedures.md so vendors attach justifications to issue instead of email; also removed stale Jenkins and ubi7 references
• updated documentation/Reference.md to remove stale Jenkins, greylist, and emailing-spreadsheet references
• updated documentation/DSOP_Duplication.md to note that Jenkins is no longer in use
• updated documentation/scan_results/ to reference updated ubi8 justification examples
  • deleted additional_justifications.md (outdated; examples not great)
• deleted documentation/gitlab-mr.sh (outdated; not sure why it's here)
• deleted documentation/ubi7.greylist.before (outdated; greylists no longer in use)