Docs: Managed service accounts disclaimer and config option

[gamab]

What this PR does / why we need it:

This PR documents the managed_service_accounts_enabled configuration option that is used in place of the externalServiceAccounts feature toggle starting with 11.3.0 (https://github.com/grafana/grafana/pull/93384)

Following https://github.com/grafana/grafana/issues/91844, I thought we could make it extra clear that we only support single organization setups.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Associated with: https://github.com/grafana/grafana/pull/93883

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✨ This PR can be merged. It will not be considered when calculating future versions of the npm packages and will not appear in the changelogs.

[gamab]

I'll GA the externalServiceAccounts feature then re-open this one for review without the feature toggle line.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had activity in the last 90 days. It will be closed in 2 weeks if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!

[github-actions[bot]]

This pull request has been automatically closed because it has not had activity in the last 2 weeks. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!