[DOC] Add document for OSD admin

[yubonluo]

What do you want to do?

OpenSearch Dashboard (OSD) admin is a new role related to workspaces that has permissions for all workspaces and saved objects inside OpenSearch.

Tell us about your request. Provide a summary of the request and all versions that are affected.
The function has merged in 2.15.
If you want to config OSD admin, you just need to modify the opensearch_dashboards.yml
Set the backend roles in groups or users, whoever has the backend roles or exactly match the user ids defined in this config will be regard as OSD admin.

opensearchDashboards.dashboardAdmin.users: ["username"]
workspace.enabled: true
savedObjects.permission.enabled: true

For example:
If your login username is wsuser, you can use wsuser as OSD admin by setting the following

opensearchDashboards.dashboardAdmin.users: ["wsuser"]
workspace.enabled: true
savedObjects.permission.enabled: true

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
PR: opensearch-project/OpenSearch-Dashboards#6554

[hdhalter]

Hi @yubonluo , can you please create the documentation PR for this update? We require a draft doc PR in order to meet entrance criteria for a viable release candidate, which starts today at 4 pm PDT. Tagging @SuZhou-Joe for tech review of the content. Let me know if you need further support. Thanks so much!

[hdhalter]

There is no user-facing implications for 2.15, so saving this for a future release.