Enhance Kubernetes job manager configuration details

- Expanded the section on Kubernetes and OpenShift job manager configuration, detailing how to size deployments, calculate parallelism and completions, and provided examples for scaling out with multiple SJM instances.
- fix formatting
- Update job-manager-configuration.mdx

fix: formatting and content

- remove duplicate content
- stick with one unit type (minutes) for $D_{avg,m}$
- reorganize headings by VM and K8s
- add content around fixed-capacity values and how that improves
  maintainability by scaling Helm releases instead
- fix mdx errors
- use \cdot for multiplication

Author: keegoid-nr