Update OpenBao chart doc for Beta

Summary

The following documentation pages need to be updated as GitLab Secrets Manager and the OpenBao chart transition from the experimental phase to beta:

• https://docs.gitlab.com/charts/development/openbao/
• https://docs.gitlab.com/charts/charts/openbao/

Implementation details

• Update https://docs.gitlab.com/charts/charts/openbao/
  • Replace experimental notice with beta notice.
  • Merge development docs into user facing docs.
• Update https://docs.gitlab.com/charts/development/openbao/
  • Link to resource considerations and expected specs guidance (output of #568356).
  • Provide scaling guidance on Horizontal Pod Autoscaler (HPA) tuning and HA.

Links

gitlab-org/cloud-native/charts/openbao#18 (comment 2790162737)