Add docs detailing our ASF Jenkins usage

[gerlowskija]

Hopefully these docs will answer a lot of questions I had when troubleshooting a recent outage of our ASF Jenkins agents.

Checklist

Please review the following and check all that apply:

• [x] I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• [x] I have created a Jira issue and added the issue ID to my pull request title.
• [x] I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• [x] I have developed this patch against the main branch.
• [x] I have run ./gradlew check.
• [x] I have added documentation for the Reference Guide