Add automatic job-pruning documentation
Before raising this MR, consider whether the following are required, and complete if so:
[ ] Unit tests[ ] Metrics- Documentation update(s)
If not required, please explain in brief why not.
Description
This documents the automatic job pruning functionality in SQL Database section in the Configuration doc (source/user/configuration.rst) an shows an example configuration.
Validation
Rendered output: https://buildgrid.gitlab.io/-/buildgrid/-/jobs/1058814494/artifacts/documentation/user/configuration.html#automatic-job-pruning
Issues addressed
Related to #367.