Add podMonitor documentation to OpenBao

What does this MR do?

Adds PodMonitor configuration documentation to enable users to scrape metrics with Prometheus Operator.

Related issues

Depends on gitlab-org/cloud-native/charts/openbao!54 (merged), issue gitlab-org/cloud-native/charts/openbao#31 (closed)

Author checklist

For general guidance, please follow our Contributing guide.

Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened. - Not applicable

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.