Create OpenBao/GitLab Secrets Manager administration documentation

Summary

Create OpenBao administration guidance page in GitLab docs covering how GitLab admin would install, configure and use the Secret Manager.

Structure overview

Administration Documentation:

• Install section - link to https://docs.gitlab.com/charts/charts/openbao/ and note for any required dependencies, general component overview and how it aligns with GitLab components
• Sizing recommendations - resource requirements and scaling guidance (depends on #568356)
• Operational considerations - upgrade, backup - cross-linking to the main relevant docs noting that existing processes should work as it is. For backup - consider moving https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/doc/development/openbao/_index.md#backup-and-restore to Admin doc instead. UPDATE: This has moved to https://docs.gitlab.com/charts/charts/openbao/#back-up-openbao.
• Health check, monitoring, and troubleshooting - related to runbooks (informed by #554040 (closed))

Note: no direct RA docs are necessary as the component is optional. At GA stage it can be added as a section in https://docs.gitlab.com/install/next_steps/ which is cross-linked from RA.

Chart Documentation Updates are tracked at #573278

Exit criteria

Self-managed user can follow documentation to install, configure and maintain OpenBao in their self-managed instance.

Background

OpenBao chart documentation currently exists in the charts repository (development docs), but comprehensive administration documentation is needed in the main GitLab documentation site before Beta/GA launch.

This follows the established pattern where components have:

• General admin documentation in docs.gitlab.com/administration/ (like Gitaly or Zoekt )
• Chart-specific configuration docs in docs.gitlab.com/charts/charts/ (like gitlab-zoekt or Gitaly chart)