[DCV-3996] Update secrets backend docs for direct backend selection

[gams87]

Updates the secrets backend documentation to match the new direct backend selection (DCV-3996):

• AWS Secrets Manager and Azure Key Vault how-tos now describe selecting the backend directly on the project or environment (no more "secondary backend" chaining), and the datacoves- prefix requirement for external backends is removed.
• Documents the environment-level override, including forcing the Datacoves-managed backend.
• Adds a section to the send-emails how-to with a test DAG to verify the smtp_default connection.
• Clarifies when the Secrets admin menu entry is visible.

Pending: two screenshots need regenerating to show the new selector (will update before merge if needed).

Companion to datacoves/datacoves#1596.

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