Document development process to setup OpenBao with GET

We wan't to enable grouppipeline security to self-serve validation of OpenBao in Cloud Native envionments via GET (gitlab-org/gitlab#568357).

Currently, GET does not expose the configuration for the OpenBao chart, but users can already enable it using a Helm custom config.

Acceptance Criteria

• Document how to setup GET for OpenBao before https://gitlab.com/gitlab-org/gitlab-environment-toolkit/-/issues/1138 is implemented.
  • Note: This can be done as a comment in this issue, no persistent/polished user-facing docs are required. End-user facing docs will be done in https://gitlab.com/gitlab-org/gitlab-environment-toolkit/-/issues/1138.
• Do a recording to demonstrate the setup if needed